Create a new project by going to File | New Project to bring up the Project Wizard window. Next, click on the Create new Project tab and set the Project Directory as you want, as we can see in the following screenshot:

As we can see from the preceding screenshot, we don't import any Unity assets package because we won't be using any of those packages in this project. This helps to reduce the size of the project because we don't have any unnecessary assets. We also set up the default of Unity mode to use 2D instead of 3D, which basically will set the camera to use the 2D view instead of the 3D view. It also sets the default asset importing to use Sprite instead of the Texture type.

Tip

By setting the default mode to 2D or 3D in Unity editor, we can go to Edit | Project Settings | Editor and change the Default Behavior Mode to 2D or 3D, as we can see in the following screenshot:

We can also switch between the 2D and 3D views by clicking on the 2D tab in the Scene view, as shown the following screenshot:

Then, go to Assets | Import Package | Custom Package… and select the Chapter1Package.unitypackage file from the folder that we just unzipped and click on Import to import all assets, as we can see in the following screenshot:

To make it easy for us, we will go to SimplePlatform | Scenes in the Project view. We will see there is one scene in this folder named SimplePlatform; double-click to open it as shown in the following screenshot:

We will see that nothing in the Hierarchy has changed. No worries, we will use this scene as our base scene to save all the progress that we will make in this project.

At the last step, we will go to SimplePlatform | Resources | Sprites | Level and click on the Background file inside this folder in the Project view. Then, we will go to the Inspector view, set it as the following screenshot, and click on Apply:

As we can see, this time we set the Sprite Mode to Single. This means that this file only contains one sprite. Now we have finished the setup part. Next, we will start building the level.

Let's go to Edit | Project Settings | Tags and Layers to bring up the Tags and Layers inspector view and set it as follows:

Tags make it easy to find the object, we need GameObject.FindWithTag("TagName") or gameObject.tag. More details on tags can be found at http://docs.unity3d.com/Documentation/Components/Tags.html.

Next, we create the background by going to SimplePlatform | Resources | Sprites | Level in the Project view, click on Background file, and drag it to the Hierarchy view, as shown in the following screenshot:

Then, we click on the Background object in the Hierarchy view, go to its Inspector view, and set the parameters as follows:

Next, we will create the Level empty game object to contain our level by going to GameObject | Create Empty; name it Level, and reset all the transformation to the default value by going to its Inspector view and right-clicking on the gear wheel icon and choosing Reset, as shown in the following screenshot:

Then, we will add the floor and wall by going to SimplePlatform | Resources | Sprites | Level in the Project view and clicking on the arrow in front of the Platform file. Then, we will click on the Floor object and drag it inside the Level game object in the Hierarchy view, and do the same thing with the Wall object, as shown in the following screenshot:

Next, we will click on the Floor object in the Hierarchy view and add Box Collider 2D by going to Component | Physics2D | Box Collider 2D. Then, we will go to Inspector and set the values of the attributes as follows:

Then, we duplicate the Floor object by pressing Ctrl + D (on Windows) or command + D (on Mac) and set the second Floor object and change the transform position as follows:

Next, we will set the Wall object, click on this object, and add the Box Collider 2D by going to Component | Physics2D | Box Collider 2D. Then, we will go to Inspector and set it as follows:

Then, we duplicate the Wall object by pressing Ctrl + D (on Windows) or command + D (on Mac) three times and set the second, third, and fourth Wall object and change the transform position as follows:

Now, we will add LargePlatform. Let's go to SimplePlatform | Resources | Prefabs in the Project view, and click on the arrow in front of the LargePlatform prefab. Then, we will click on the Platform_5, Platform_4, Platform_6, and Platform_1 objects and set Tag and Layer to Ground. Then, we will drag LargePlatform inside the Level game object in the Hierarchy view, go to its Inspector, and set it as follows:

Next, we add Platform_1. Let's go to SimplePlatform | Resources | Prefabs in the Project view, click on the Platform_1 object, and set Tag and Layer to Ground. We will see the Change Layer pop up; click on the No, this object only button. Then, we will drag it inside the Level game object in the Hierarchy view, go to its Inspector, and set it as follows:

Then, we do the same thing for Platform_3. Let's go to SimplePlatform | Resources | Prefabs in the Project view, click on the Platform_3 object, and set Tag and Layer to Ground. We will see the Change Layer pop up; click on the No, this object only button. Then, we will drag it inside the Level game object in the Hierarchy view. Go to Inspector, and set it as follows:

Next, we will duplicate Platform_3 by pressing Ctrl + D or command + D to create the second Platform_3 object and set it as follows:

We will see the level in the Scene and Hierarchy view similar to the following screenshot:

We can see that we already have the Platform_1 and Platform_3 prefab. Now, we will create the Platform_2 prefab from scratch. First, we will go to SimplePlatform | Resources | Sprites | Level in the Project view and click on the arrow in front of the Platform file. Then, we will click on the Platform_2 object and drag it inside the Level game object in the Hierarchy view. Click on this object and add Polygon Collider 2D by going to Component | Physics2D | Polygon Collider 2D. Then, we will go to Inspector and set the values of the attributes as follows:

Now, we go to the Scene view. We will see that our Platform_2 sprite has Polygon Collider 2D that matches the shape of this sprite, as shown in the following screenshot:

From the preceding screenshot, we will see that there are too many vertices for this sprite. So, we need to decrease and adjust the vertices to match the sprite.

Next, we will adjust the vertices in Polygon Collider 2D. To make it easy to adjust, we will go to Platform_2 Inspector and click on the arrow in front of Sprite Renderer to hide the 2D outline gizmos, as shown in the following screenshot:

Now, we will remove the vertices in Collider Info from 50 to 7, which will optimize the content and help the overall performance in the scene. We can do this by pressing Ctrl (on Windows) or command (on Mac) . We will see the red dot. We can now click on the vertex to remove the vertices from 50 to 7, similar to the following screenshot:

Next, we will adjust the vertices to match the sprite by pressing Shift. We will see the green dot. We can go to each vertex to move it.

We will get the result as shown in the following screenshot (the position of each vertex shows as the circle):

Then, we will create the Edge game object, which is basically to prevent the character from getting stuck on the platform edge. Let's go to GameObject | Create Empty, name it Edge, and drag it inside Platform_2.

Then, we will add Box Collider 2D to the Edge object by going to Component | Physics2D | Box Collider 2D. Go to Inspector and set it as follows:

Next, we will duplicate the Edge game object by pressing Ctrl + D or command + D to create the second Edge object and set the values of the attributes as follows:

We got our Platform_2 game object. Next, we will create the prefab of this game object by dragging the Platform_2 game object from the Hierarchy view to the SimplePlatform | Resources | Prefabs in the Project view.

Next, we will create two more Platform_2 objects. Let's press Ctrl + D or command + D twice to create the second and third Platform_2 object and change the transform position as follows:

Finally, we will see the result of our level as the following screenshot, and we are now ready for the next step:

Now, we will go to SimplePlatform | Resources | Sprites | Characters and click on the Spritesheet file inside this folder in the Project view. Then, we will go to the Inspector view, set it as shown the following screenshot, and click on Apply:

In the Inspector view, we click on the Sprite Editor button, which will open the Sprite Editor window. Then, we click on the Slice drop-down button to bring up the Slice window. We will leave the parameters as it is and then click on the Slice button to slice all the sprites as the following screenshot:

Now, we will see the white frame on each sprite, click on the first sprite (on the top-left corner) to bring up the sprite window, and rename from Spritesheet_0 to Walk_0, as shown in the following screenshot:

Next, we will keep going by clicking on each sprite from top-left to bottom-right corner and rename the sprites as follows:

Then, we will click on the Apply button at the top-right corner of the Sprite Editor window, as shown in the following screenshot:

If we go to the Project view, we will see that the Spritesheet file has an arrow in front of it. If we click on the arrow, we see all the sprites that we just renamed, as shown in the following screenshot:

Finally, we will go to SimplePlatform | Resources | PhysicsMaterials in the Project view. We will see that there is Physics2D Material for Ground and Edge, which control Friction and Bounciness between Player and Level. We still need to create one more Physics2D Material player for our player. So, right-click and go to Create | Physics2D Material, name it Player, and set Friction to 1.2 and Bounciness to 0, as shown in the following screenshot:

Let's go to SimplePlatform | Resources | Sprites | Characters in the Project view and click on the arrow in front of the Spritesheet file. Then, we will click on both the Idle_0 and Idle_1 objects and drag it to the Hierarchy view. We will see the pop-up window asking to create a new animation, set the filename to Idle, put it in SimplePlatform | Resources | Sprites | Animation, and click on Save, as shown in the following screenshot:

Next, we will go to SimplePlatform | Resources | Sprites | Animation and click on the Idle_0 object and rename it to PlayerAnimator.

Then, we go back to SimplePlatform | Resources | Sprites | Characters in the Project view and click on the arrow in front of the Spritesheet file. After that, we will click on all Walk objects from Walk_0 to Walk_7 (eight objects in total), and drag them to the Idle_0 game object in the Hierarchy view. We will see the pop-up window asking us to create the new animation again, set the filename to Walk, put it in SimplePlatform | Resources | Sprites | Animation, and click on Save.

In the Spritesheet file in the Project view, we will click on Jump_0 and Jump_1 and drag them to the Idle_0 game object in the Hierarchy view. We will see the pop-up window asking us to create the new animation again, set the filename to Jump, put it in SimplePlatform | Resources | Sprites | Animation, and click on Save.

We will do the same as the last step, but this time we will click on Jump_1 and Jump_2, and drag them to the Idle_0 game object in the Hierarchy view. We will see the pop-up window asking us to create the new animation again, set the filename to Fall, put it in SimplePlatform | Resources | Sprites | Animation, and click on Save.

Next, we will go to SimplePlatform | Resources | Sprites | Animation in the Project view. Then, we will click on the Fall and Jump animation, go to their Inspector view, and uncheck Loop Time for both clips, as shown in the following screenshot:

Then, we go to the Hierarchy view, rename Idle_0 to Player, go to its Inspector view, and set the values of the attributes as follows:

Next, we will set up PlayerAnimator. Let's go to SimplePlatform | Resources | Sprites | Animation in the Project view and double-click the PlayerAnimator object to bring up the Animator view. Then, we click on each clip and layout the position similar to the following screenshot:

Then, we click on each clip and go to Inspector to set the speed of each animation clip. We will start with the Idle clip and move ahead; let's go to each clip inspector and set it up as follows:

Next, we will add a few parameters to control when each animation will be played. Let's go to the Parameters tab at the bottom-left corner of the Animator view, click on the plus icon four times, and choose the name and type of parameter as follows:

These parameters will be used as a condition to control the changing state of each animation clip on our character.

Next, we will create the transition between each animation state. First, we will start with our base animation Idle and transition to Walk. Right-click on the Idle clip and choose Make Transition, which will bring up the arrow, and click-and-drag again on top of the Walk clip. We will see the arrow line link from the Idle clip to the Walk clip, as shown in the following screenshot:

Then, we want to click on the arrow line that we just created and go to Inspector | Conditions and set Speed to a value greater than 0.1.

Next, we will create the transition back from the Walk to Idle animation. So, let's right-click on the Walk clip, choose Make Transition, and then click-and-drag on Idle. Then, we will click on the arrow line again to go to the Inspector and set a value less than 0.1 for Speed in the Conditions view.

Then, we set up the rest of the animation. This time we want the transition from Walk to Fall. So, let's right-click on the Walk clip, choose Make Transition, and then click-and-drag on Fall. After that, we will click on the arrow line again to go to Inspector and set the Conditions view to Fall.

Also, we will create the transition from Fall back to Walk, go to the transition Inspector, click on the plus icon to add another condition and set Speed as well as Ground to a value greater than 0.1.

Next, we want the Idle clip transition to Fall. Let's create the transition arrow from Idle to Fall, go to Inspector, and set the Conditions view to Fall.

We also need to create the transition from Fall back to Idle, set up its Inspector view, click on the plus icon to add another condition, and set Speed to a value less than 0.1.

Then, we will create the transition arrow from Idle to Jump and set up its Inspector view by setting its Conditions view to Jump.

Next, we create the transition arrow from Walk to Jump and set up its Inspector view by setting its Conditions view to Jump.

Then, we need the transition arrow from Jump to Fall and set up its Inspector view by setting its Conditions view to Fall.

The current Animator view will look similar to the following screenshot:

Finally, we will need to attach Rigidbody 2D and Box Collider 2D to our Player game object. Let's go back to the Hierarchy view, click on the Player game object, go to Component | Physics 2D | Rigidbody 2D to add Rigidbody 2D, go to Component | Physics 2D | Box Collider 2D to add Box Collider 2D, and then set up Inspector as follows:

Next, we want to make sure that Unity uses MonoDevelop as our main Scripting editor (Unity | Preferences in Mac or Edit | Preferences in Windows).

We will see a Unity Preferences window. In the External Tools tab, go to the External Script Editor and make sure that the MonoDevelop option is selected. Click on Browse…, go to Applications | Unity | MonoDevelop.app in Mac or {unity install path} | Unity | MonoDevelop | MonoDevelop.exe in Windows, and we are done:

First, go to Assets | Create | Javascript (for Unity JavaScript developers) or Assets | Create | C# Script (for C# developers) and name our script as PlayerController_2D.

Double-click on the script; it will open the MonoDevelop window.

Now, we will see three windows in the MonoDevelop screen:

Let's get our hands dirty with some code—first start with defining the following functions to initialize the variables: the Awake() and Start() function.

We need to remove the autogenerated code and replace it with the following code:

// Unity JavaScript user:

#pragma strict
@script RequireComponent(AudioSource)
@script RequireComponent(BoxCollider2D)
@script RequireComponent(Rigidbody2D)
// Distance from the character position to the ground
private final var RAYCAST_DISTANCE : float = 0.58f;
// This number should be between 0.35 to 0.49
private final var CHARACTER_EDGE_OFFSET : float = 0.40f;	
var doorOpenSound : AudioClip;
var getKeySound : AudioClip;
var jumpSound : AudioClip;
var moveForce : float = 80f;
var jumpForce : float = 350f;
var maxSpeed : float = 3f;
var layerMask : LayerMask;
private var _animator : Animator;
private var _boxCollider2D : BoxCollider2D;
private var _isFacingRight : boolean = true;
private var _isJump : boolean = false;
private var _isFall : boolean = false;
private var _isGrounded : boolean = false;
private var _gameEnd : boolean = false;
private var _height : float;
private var _lastY : float;
private var _horizontalInput : float;
function Awake() {
  _animator = GetComponent.<Animator>();
  _boxCollider2D = GetComponent.<BoxCollider2D>();
  _height = _boxCollider2D.size.y;
}
function Start () {
  _isFacingRight = true;
  _isJump = false;
  _isFall = false;
  _isGrounded = false;
  _gameEnd = false;
  _lastY = transform.position.y;
  Camera.main.transform.position = new Vector3(transform.position.x, transform.position.y, Camera.main.transform.position.z);
}

// C# user:

using UnityEngine;
using System.Collections;
[RequireComponent(typeof(AudioSource))]
[RequireComponent(typeof(BoxCollider2D))]
[RequireComponent(typeof(Rigidbody2D))]
public class PlayerController_2D : MonoBehaviour
{
  // Distance from the character position to the ground
  const float RAYCAST_DISTANCE = 0.58f;
  // This number should be between 0.35 to 0.49
  const float CHARACTER_EDGE_OFFSET = 0.40f;
  
  public AudioClip doorOpenSound;
  public AudioClip getKeySound;
  public AudioClip jumpSound;
  public float moveForce = 80f;
  public float jumpForce = 350f;
  public float maxSpeed = 3f;
  public LayerMask layerMask; 
  Animator _animator;
  BoxCollider2D _boxCollider2D;
  bool _isFacingRight = true;
  bool _isJump = false;
  bool _isFall = false;
  bool _isGrounded = false;
  bool _gameEnd = false;
  float _height;
  float _lastY;
  float _horizontalInput;
  void Awake() {
    _animator = GetComponent<Animator>();
    _boxCollider2D = GetComponent<BoxCollider2D>();
    _height = _boxCollider2D.size.y;
  }
  void Start () {
    _isFacingRight = true;
    _isJump = false;
    _isFall = false;
    _isGrounded = false;
    _gameEnd = false;
    _lastY = transform.position.y;
    Camera.main.transform.position = new Vector3(transform.position.x, transform.position.y, Camera.main.transform.position.z);
  } 
}

Next, we will add the Flip() function after the Start() function to make our character sprite show the correct graphics when moving left or right. The code for Unity JavaScript and C# users are as follows:

// Unity JavaScript user:

private function Flip () {
  _isFacingRight = !_isFacingRight;
  var scale : Vector3 = transform.localScale;
  scale.x *= -1;
  transform.localScale = scale;
}


// C# user: (Put the code inside the class)

void Flip () {
  _isFacingRight = !_isFacingRight;
  Vector3 scale = transform.localScale;
  scale.x *= -1;
  transform.localScale = scale;
}

Next, we will add another function, which will use Physics2D.Raycast to check whether the player is on the ground or not. Let's create the Grounded() function as follows:

// Unity JavaScript user:

private function Grounded () {
  var distance : float = _height*RAYCAST_DISTANCE;
  var hitDirectionV3 : Vector3 = transform.TransformDirection(-Vector3.up);
  var hitDirection : Vector2 = new Vector2(hitDirectionV3.x,hitDirectionV3.y);	
  var rightOrigin : Vector2 = new Vector2(transform.position.x + (_boxCollider2D.size.x*CHARACTER_EDGE_OFFSET), transform.position.y);
  var leftOrigin : Vector2 = new Vector2(transform.position.x - (_boxCollider2D.size.x*CHARACTER_EDGE_OFFSET), transform.position.y);
  var origin : Vector2 = new Vector2(transform.position.x, transform.position.y);
  if (Physics2D.Raycast (origin, hitDirection, distance, layerMask.value)) {
    _isGrounded = true;
  } else if (Physics2D.Raycast (rightOrigin, hitDirection, distance, layerMask.value)) {
    _isGrounded = true;
  } else if (Physics2D.Raycast (leftOrigin, hitDirection, distance, layerMask.value)) {
    _isGrounded = true;
  } else {
    if (_isGrounded) {
      if (Mathf.Floor(transform.position.y) == _lastY) {
        _isGrounded = true;
      } else {
        _isGrounded = false;
      }
    }
  }
  _lastY = Mathf.Floor(transform.position.y);
}


// C# user: (Put the code inside the class)

void Grounded () {
  float distance = _height*RAYCAST_DISTANCE;
  Vector3 hitDirectionV3 = transform.TransformDirection (-Vector3.up);
  Vector2 hitDirection = new Vector2(hitDirectionV3.x,hitDirectionV3.y);
  Vector2 rightOrigin = new Vector2(transform.position.x + (_boxCollider2D.size.x*CHARACTER_EDGE_OFFSET), transform.position.y);
  Vector2 leftOrigin = new Vector2(transform.position.x - (_boxCollider2D.size.x*CHARACTER_EDGE_OFFSET), transform.position.y);
  Vector2 origin = new Vector2(transform.position.x, transform.position.y);
  if (Physics2D.Raycast (origin, hitDirection, distance, layerMask.value)) {
    _isGrounded = true;
  } else if (Physics2D.Raycast (rightOrigin, hitDirection, distance, layerMask.value)) {
    _isGrounded = true;
  } else if (Physics2D.Raycast (leftOrigin, hitDirection, distance, layerMask.value)) {
    _isGrounded = true;
  } else {
    if (_isGrounded) {
      if (Mathf.Floor(transform.position.y) == _lastY) {
        _isGrounded = true;
      } else {
        _isGrounded = false;
      }
    }
  } 
  _lastY = Mathf.Floor(transform.position.y);
}

Next, we will include the script in the Update() function and LateUpdate() function that get called after the Update() function. These functions will be used to check the input from the user and update the camera to follow our character movement. The code for these functions are as follows:

// Unity JavaScript user:

function Update() {
  if (!_gameEnd) {
    _horizontalInput = Input.GetAxis("Horizontal");
    if ((!_isFacingRight && (_horizontalInput > 0)) || 
      (_isFacingRight && (_horizontalInput < 0))) {
      Flip();
    }
    Grounded();
    if(Input.GetButtonDown("Jump") && _isGrounded) {
      _isJump = true;
    }
  }
}
function LateUpdate() {
  if (!_gameEnd) {
    Camera.main.transform.position = new Vector3(transform.position.x, transform.position.y, Camera.main.transform.position.z);
  }
}


// C# user: (Put the code inside the class)

void Update() {
  if (!_gameEnd) {
    _horizontalInput = Input.GetAxis("Horizontal");
    if ((!_isFacingRight && (_horizontalInput > 0)) || 
      (_isFacingRight && (_horizontalInput < 0))) {
      Flip();
    }
    Grounded();
    if(Input.GetButtonDown("Jump") && _isGrounded) {
      _isJump = true;
    }
  }
}
void LateUpdate() {
  if (!_gameEnd) {
    Camera.main.transform.position = new Vector3(transform.position.x, transform.position.y, Camera.main.transform.position.z);
  }
}

Then, we create a FixedUpdate() function, which will handle the animation state changing (Idle, Walk, Jump, or Fall) and add force to move our character horizontally and vertically. Let's add this function as follows:

// Unity JavaScript user:

function FixedUpdate () {
  if (!_gameEnd) {
    _animator.SetFloat("Speed", Mathf.Abs(_horizontalInput));
    var xSpeed : float = Mathf.Abs(_horizontalInput * rigidbody2D.velocity.x);
    if (xSpeed < maxSpeed) {
      rigidbody2D.AddForce(Vector2.right * _horizontalInput * moveForce);
    }
    if (Mathf.Abs(rigidbody2D.velocity.x) > maxSpeed) {
      var newVelocity : Vector2 = rigidbody2D.velocity;
      newVelocity.x = Mathf.Sign(newVelocity.x) * maxSpeed;
      rigidbody2D.velocity = newVelocity;
    }
    if(_isJump) {
      _animator.SetTrigger("Jump");
      audio.volume = 0.3f;
      audio.PlayOneShot(jumpSound);	
      rigidbody2D.AddForce(new Vector2(0f, jumpForce));   
      _isJump = false;
    }
    if (!_isGrounded) {
      if ((rigidbody2D.velocity.y <= 0f) && !_isFall) {
        _animator.SetTrigger("Fall");
        _isFall = true;
      }
    }
    if (_isGrounded) {
      if (_isFall) {
        _animator.SetTrigger("Ground");
        _isFall = false;
      } else {
        var animationStateInfo : AnimatorStateInfo = _animator.GetCurrentAnimatorStateInfo(0);
        if ((rigidbody2D.velocity.y < 0f) && (animationStateInfo.IsName("Base Layer.Jump"))) {
          _animator.SetTrigger("Fall");
          _isFall = true;
        }
      }
    }
  }
}


// C# user: (Put the code inside the class)

void FixedUpdate () {
  if (!_gameEnd) {
    #region Setting player horizontal movement
    _animator.SetFloat("Speed", Mathf.Abs(_horizontalInput));
    float xSpeed = Mathf.Abs(_horizontalInput * rigidbody2D.velocity.x);
    if (xSpeed < maxSpeed) {
      rigidbody2D.AddForce(Vector2.right * _horizontalInput * moveForce);
    }
    if (Mathf.Abs(rigidbody2D.velocity.x) > maxSpeed) {
      Vector2 newVelocity = rigidbody2D.velocity;
      newVelocity.x = Mathf.Sign(newVelocity.x) * maxSpeed;
      rigidbody2D.velocity = newVelocity;
    }
    #endregion
    #region If the player should jump
    if(_isJump) {
      _animator.SetTrigger("Jump");
      audio.volume = 0.3f;
      audio.PlayOneShot(jumpSound);	
      rigidbody2D.AddForce(new Vector2(0f, jumpForce));	
      _isJump = false;
    }
    #endregion
    #region If the player should fall
    if (!_isGrounded) {
      if ((rigidbody2D.velocity.y <= 0f) && !_isFall) {
        _animator.SetTrigger("Fall");
        _isFall = true;
      }
    }
    #endregion
    #region If the player is grounded
    if (_isGrounded) {
      if (_isFall) {
        _animator.SetTrigger("Ground");
        _isFall = false;
      } else {
        AnimatorStateInfo animationStateInfo = _animator.GetCurrentAnimatorStateInfo(0);
        if ((rigidbody2D.velocity.y < 0f) && (animationStateInfo.IsName("Base Layer.Jump"))) {
          _animator.SetTrigger("Fall");
          _isFall = true;
        }
      }
    }
    #endregion
  }
}

We can see how to use #region ….. #endregion in the following screenshot:

Lastly, we will add the OnDrawGizmos() function in this class. It is a very nice function that allows us to debug the game, the result of which we won't see in the real game. Let's add the following block of code:

// Unity JavaScript user:

function OnDrawGizmos() {
  _boxCollider2D = GetComponent.<BoxCollider2D>();
  _height = _boxCollider2D.size.y;
  var distance : float = (_height * RAYCAST_DISTANCE);
  var rightOrigin : Vector3 = new Vector3(transform.position.x + (_boxCollider2D.size.x*CHARACTER_EDGE_OFFSET), transform.position.y, transform.position.z);
  var leftOrigin : Vector3 = new Vector3(transform.position.x - (_boxCollider2D.size.x*CHARACTER_EDGE_OFFSET), transform.position.y, transform.position.z);
  Gizmos.color = Color.red;
  Gizmos.DrawRay(transform.position, transform.TransformDirection (-Vector3.up) * distance);
  Gizmos.DrawRay(rightOrigin, transform.TransformDirection (-Vector3.up) * distance);
  Gizmos.DrawRay(leftOrigin, transform.TransformDirection (-Vector3.up) * distance);
}


// C# user: (Put the code inside the class)
void OnDrawGizmos() {
  _boxCollider2D = GetComponent<BoxCollider2D>();
  _height = _boxCollider2D.size.y;
  float distance = (_height * RAYCAST_DISTANCE);
  Vector3 rightOrigin = new Vector3(transform.position.x + (_boxCollider2D.size.x*CHARACTER_EDGE_OFFSET), transform.position.y, transform.position.z);
  Vector3 leftOrigin = new Vector3(transform.position.x - (_boxCollider2D.size.x*CHARACTER_EDGE_OFFSET), transform.position.y, transform.position.z);
  Gizmos.color = Color.red;
  Gizmos.DrawRay(transform.position, transform.TransformDirection (-Vector3.up) * distance);
  Gizmos.DrawRay(rightOrigin, transform.TransformDirection (-Vector3.up) * distance);
  Gizmos.DrawRay(leftOrigin, transform.TransformDirection (-Vector3.up) * distance);
}

Now, save it and go back to Unity; drag-and-drop the PlayerController_2D script to Player, click on Player, and go to the Inspector window. Click on Idle Sprite and Walk Sprite to expand it and then set the following:

Let's go to SimplePlatform | Resources | Sprites | Level in the Project view and drag the Key object to the Hierarchy view. At the Hierarchy view, we will click on the Key game object and then go to Component | Physics 2D | Circle Collider 2D to add the Circle Collider 2D component. Next, we will go to its Inspector view, and set it as follows:

Next, we click on the Key game object in the Hierarchy view and drag inside SimplePlatform | Resources | Prefabs in the Project view to create the Key prefab object.

Then, we will create the Door game object by going to SimplePlatform | Resources | Sprites | Level in the Project view and dragging the DoorClose object to the Hierarchy view. At the Hierarchy view, we will click on the DoorClose game object, go to Component | Physics 2D | Box Collider 2D, and then go to Component | Miscellaneous | Animator.

Next, we will click on the DoorClose game object, rename it to Door, and go to Inspector, and set it as follows:

Now we've got the Key and Door game object. We need to go back to the PlayerController_2D script and add a function to make this work. Let's open the PlayerController_2D script and add the OnTriggerEnter2D() function as shown in the following highlighted script:

// Unity JavaScript user:
   
private var _hasKey : boolean = false;
   …
function Start () {
…
  _hasKey = false;
}
…
function OnTriggerEnter2D (hit : Collider2D) : IEnumerator {
  if (!_gameEnd) {
    if (hit.tag == "Key") {
      if (!_hasKey) {
        _hasKey = true;
        audio.volume = 1.0f;
        audio.PlayOneShot(getKeySound);
        Destroy (hit.gameObject);
      }
    }
    
    if (hit.tag == "Door") {
      if (_hasKey) {
        _gameEnd = true;
        _animator.enabled = false;
        audio.volume = 1.0f;
        audio.PlayOneShot(doorOpenSound);
        var doorAnimator : Animator = hit.GetComponent.<Animator>();
        doorAnimator.SetTrigger("DoorOpen");
        yield WaitForSeconds(1);
        doorAnimator.SetTrigger("DoorClose");
        Destroy (gameObject);
      }
    }
  }
}


// C# user: (Put the code inside the class)

bool _hasKey = false;
…
void Start () {
…
  _hasKey = false;
}
…
IEnumerator OnTriggerEnter2D (Collider2D hit) {
  if (!_gameEnd) {
    if (hit.tag == "Key") {
      if (!_hasKey) {
        _hasKey = true;
        audio.volume = 1.0f;
        audio.PlayOneShot(getKeySound);
        Destroy (hit.gameObject);
      }
    }
    if (hit.tag == "Door") {
      if (_hasKey) {
        _gameEnd = true;
        _animator.enabled = false;
        audio.volume = 1.0f;
        audio.PlayOneShot(doorOpenSound);
        Animator doorAnimator = hit.GetComponent<Animator>();
        doorAnimator.SetTrigger("DoorOpen");
        yield return new WaitForSeconds(1);
        doorAnimator.SetTrigger("DoorClose");
        Destroy (gameObject);	
      }
    }
  }
}

Next, we click on the Player game object in the Hierarchy view and drag the object inside SimplePlatform | Resources | Prefabs in the Project view to create the Player prefab object.

Inside SimplePlatform | Resources | Prefabs in the Project view, we will see the RestartButton_C# and RestartButton_JS prefab. Now, we will create a new RestartButton from those prefabs, drag the RestartButton_C# prefab (for C# user), and drag the RestartButton_JS prefab (for Unity JavaScript user) to the Hierarchy view.

Then, let's click on the RestartButton prefab, go to its Inspector view, and set the values of the attributes as follows:

For the last step, we will go back to the PlayerController_2D script and the _restartButton variable to make this work. Let's open the PlayerController_2D script and add the OnTriggerEnter2D() function as shown in the highlighted script:

// Unity JavaScript user:
…
private var _restartButton : GUITexture;
…
function Awake () {
  …
  _restartButton = GameObject.FindWithTag("RestartButton").guiTexture;
}
function Start () {
  …
  _restartButton.enabled = false;
}
…
function OnTriggerEnter2D (hit : Collider2D) : IEnumerator {
  if (!_gameEnd) {
    …
    if (hit.tag == "Door") {
      if (_hasKey) {
       …
        _restartButton.enabled = true;
      }
    }
  }
}


// C# user: (Put the code inside the class)
…
GUITexture _restartButton;
…
void Awake () {
  …
  _restartButton = GameObject.FindWithTag("RestartButton").guiTexture;
}
void Start () {
…
  _restartButton.enabled = false;
}
…
IEnumerator OnTriggerEnter2D (Collider2D hit) {
  if (!_gameEnd) {
   …
    if (hit.tag == "Door") {
      if (_hasKey) {
        …
        _restartButton.enabled = true;
      }
    }
  }
}