I had a building on a tower, on a sculpted piece of land and when I switched to the first person view, I fall through the floors and down till I hit the land underneath. Is there a way to stop that? So I can "walk" around in the building?

You must move the camera into the scene so  it is over the terrain. Then First person will drop to the surface.
Move the camera and drop it with the 0 (zero) key until you hit dirt. Then you can position it so First Person rests above the ground.

If the camera is not over the terrain, it will fall into infinity.