While the above sections are a minimal but complete overview of how to set up an interactive walk around your architectural model, some additional recommendations and tips are valuable, such as:
To walk around your model, provide a terrain or at least a slab to walk on. As an alternative, you can use a Plane or Quad GameObject or, even better, a Unity Terrain.
When you notice your character cannot get up the stairs or onto a slope, you will have to adjust the properties of the Character Controller component of the FPC, such as Step Offset and Slope Limit.
When you can see through the walls when you get too close to it, lower the Near Clipping Plane of the Camera component.
If your character cannot pass through a door opening, try lowering the Radius of the Character Controller component.
Unity sometimes freezes when characters collide with geometry that is too complex, for example, plants or furniture objects with a full Mesh collider. Replace the collider with a simplified one...