This is actually pretty good. I mean, it'd nice if you could start each wave with one/all of the weapon upgrades from each level before it when you start at a later level. Also, the lack of a 'you died' or 'game over' interlude makes it feel a bit weird. The lack of healing drops, ever, also makes this a bit harder than it actually needs to be and I personally would like if you at least made one health drop every 3 levels or so.