bugs

First pic, npc clipping through wall where npc is standing, and second pic getting good amount splash damage from pillar both sides toward edges of the shelter, getting even if behind the tall wall of the shelter. 

You can put a Building topology around the arena so that there are no trees, but then there will be no grass and it won't look nice, so I didn't do it. I advise you not to place the arena near the forest. Regarding NPCs going through doors, that's a feature of Rust, even on Oilrigs NPCs can go through doors to a crate room.

ok thanks for the info and help