Invisble Faces

Hello,
I have a prblem with a easy mesh.
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I usually modelling with Blender.
Is there a way to fix this problem in Unity3d Free/Indie?

This problem occurs because your planes are one-sided. That means they can only be seen from one side. In your case, the inside. So if you’re viewing them from the outside you can’t see them. As Next Beat Games said, you should make an outside for the walls. Or you can make a shader that makes them visible from both sides.