Pacing seemed a bit slow for me, but I think it could be great with a few minor tweaks. When I reached round 10 there where no enemies spawning so I really continue. Would have loved to try some of the special upgrades, but I don't have it in me to slowly grind my way there again.
Also I feel like the Escape key (the one on my keyboard) should be doing something in this game.
But anyway, good job guys!