Really solid, straightforward and fun to play. It has some more annoying mechanics (like the arcing projectile and distance changing from level to level, making aim delay adjustment varied or the splintering shards getting slightly in the way of extra damage to unharmed enemies perk), but it's cool overall.
I'd consider putting up a gold counter somewhere on the screen, just so you don't have to enter upgrades screen just to check the total gold and maybe a visual representation of firing cooldown (maybe the aiming circle filling up or something). It's not a huge deal, especially for a game that splits into minute-long levels, but it's certainly some degree of QOL.