Very, very poorly balanced. Crits are kinda pointless since you'll often be one-shotting things, attack speed has the same issue, and the bats (fully half of the upgrade tree?!) are completely useless because the spawn rate and damage are so low. Once you max out the few useful upgrades (time, damage, walk speed) it's basically just a matter of retrying a few times to get an enemy layout that minimizes walking enough.

There's the kernel of a decent game here, but right now it's just not worth playing.