I like the competitive nature of this game. I do however see a problem with people just camping a few enemies and waiting for them to respawn. To mitigate this, I would make them respawn only after a certain number of them are defeated. Also there is a bug - when Esc is pressed, it disables player control and doesn't open the pause menu. I'd also make the walls around thicker to make it feel more like an arena (and less like a floating box :D).