The first problem is that http://i.imgur.com/8FV4h0p.jpg. I placed a small collision box directly underneath the turret. No effect.

The second problem is that http://i.imgur.com/XYZi3eJ.jpg. I placed a long collision box that covers the entire rear wall. No effect. EDIT: Solved with a portal box.

I probably created those collision boxes incorrectly. What am I doing wrong?