There is also a little secret that is a bit hard to find without noclip.
You can actualy get in without noclip, but I clasify it as a "wanted bug". Teleporter that send you in the secret is behind a wall, but it seems there's no way to dicover it. You can, but after you find out the wall the teleporter is behind is nonsolid... You can get through the wall in the teleporter room...