Chaotic, but very fun! In the beginning I've tried to delay building turrets as much as possible, but placing four basic ones at each corner was a better strategy, since I've found that enemies are not necessarily spawning at the edge (no pun intended) of the level, sometimes they appear quite close to the "base". I've had loads of fun, even though one time I've selected a machine gun turret (cost: 120 time resources) by mistake and commenced placing it even though I haven't had enough dough to afford it and.. yep, that killed me instantly :) maybe you could add a basic test for the current time resources so it's not possible for the player to screw themselves that easily :D
Anyway, I love the graphics and the audio is quite interesting as well! A bit chaotic maybe, sometimes it feels a bit async (or maybe it's just polyrhythmic? I've flunked out of music school :). The atmosphere is great, it felt a bit like playing Darwinia (which I loved). Really nice job overall!