Love the game! It's really addicting :P
By the way I found an interesting interaction. If you have the game running on a tab, then but swap to a different tab, the game will pause, the timer will stop, but enemies will still spawn. So when you tab back in all those enemies start closing in on you at once. That said, after a couple hours of accidentally spawning critters, a big ring of death popping in to jump you is pretty funny.
Notably this does not happen if you just focus on a different window, you have to be looking at a different tab. (This was discovered on Firefox, I don't know if it happens on Chrome too)