Are your walls moving? Otherwise you can set POLYGON flag for them so they have better collision. Tapping f11 twice with #include <default.c> included will allow you to check collision. Also where do you set the invisible flag for the wall? One more thing, you are not using WED invisible blocks for your walls right , but just models right? Cause for when you want to make WED blocks invisible but still have collision you will have to set their surfaces to not shaded (instead of marking the invisible flag for them).