This has been talked about before but I can't find the map. This spot should be illegal. It's crouch entry. No debate there.
It's actually possible to get in there without crouching which is why I believe Haplo didn't include this spot on the list. PM for details on how to get it done.
The only way to get in without crouching requires you to jump and try to slide in. Most of the time it's nearly impossible if you have 3-4 CTs sitting there autofiring out the door. Personally, I say it's illegal and I'll keep telling people to get out of the spot.
It's pretty easy to get in there as a zombie you just crouch jump to it. I've done it many times to kill people in there. Probably why it wasnt made illegal when haplo was making the illegal spots post on the forums for that map. Also its a bit of a easter egg in the map. Its like oh hey theres a spot inside the wall omgs. Since most spots like that in other maps arent illegal since you have to know where its at and how to get in.