Wooh, discovered all secrets!
Also: I find the level design interesting.
For example, there is that one area with a bridge. On first pass you (probably) need to shoot the slime which is on the other side. When crossing the room the other way around, you can easily pass the slime.
There are other rooms that, I suspect, have intentionally avoidable enemies. There are also some room which force you to battle, though.