New to Blender. When importing to unity, something happens to the model?

As I said, I just started learning how to model about 4 days ago, and i’m pretty sure this is a rookie mistake.

Here is the image. First one is what it looks like in blender, second is what happens to the model in unity.

Now, when I export .FBX file and import it into unity, the walls on the inside dissapear, and it looks like nothing is there. When you walk around the model, the walls will continue to appear/disappear. Anyone know how to fix this?

Keep in mind that Unity can handle .blend files natively.