I dont really know what's happening with the walls of my model but there is the video

http://www.xfire.com/video/4c3291/

(it was my first model i am aware of the weird size and texture problems now)

your problem is that your walls are made of a single polygon. battefront renders normal polygons only from one side to save performance. you should duplicate the walls, move them inwards a bit and mayb escale them a bit smaller. then select all the polygons of the insidewalls, right click and choose "invert polygons", that will make them face inwards.

ok that makes sense i guess that is what i should do for every interior of every building

EDIT: sorry to bump the topic but i thought i had a good question (at least for someone with my modeling skill) and it is why do i have to scale down the duplicated polygons? if i invert them they are only viewed from one side ingame so the z fighting issue would not matter or would it? I have been doing your suggested method but it can get annoying sometimes.