Simply Stunning
Well, I'm sure you're sick of hearing this by now (or maybe not, I dunno), but this is quite honestly one of the best TDs I've played in...ever. The graphics are simple but beautiful, the enemies are diverse and inspire a sense of accomplishment upon killing them, the levels are challenging but not impossible, and the towers are quite easily the most fun to upgrade I've ever had.
All that said, please don't take this next part as harsh criticism, just suggestions that might make a fun game even more fun. A create-your-own-level mode, or an endless mode, would be a fantastic addition. Also, a way of sharing upgrade nodes, maybe a sort of memory-sharing system, would be pretty cool. Like say putting an Infector down and filling up it's upgrades, then putting a Compressor right below it and having it share the upgrades installed on the Infector, and then a Scanner below THAT, also sharing, etc etc. Deleting one "linked" program would require the deletion of all the linked programs, but I think it would add another level of customization to the upgrading of towers.
TL;DR,
Fantastic game, and I hope you plan on more eventually.
