playme3000

I just started mapping yesterday. Everything went pretty smooth thanks to following daddy dirk dirk and RingoD123's tutorial. I'm pretty much finished but when testing i've notice i can walk straight through the bunker walls (those grass inclines for the sides). I've noticed an "invisible" tileset and assumed that would be invisible walls but turns out they aren't. Anything i'm doing wrong or any ideas on how to prevent people from just walking into the wall?