You unlock the the ultimate towers by having a max upgrade of each unit type respective to ranged or melee. I assume this includes both upgrade paths (if applicable).
Overall I find this a great game, and I'd love to see badges added for it. It can run a little sluggishly in later levels on lower spec computers, but it's far better performance than most defense games out there. The sound can be quite annoying too at later levels, but again, no worse than most others available. I give it a 4/5 overall.
This is a good game with good design, it however lacks goal. On all maps the game becomes incredibly easy and is nothing more than clicking buttons and waiting for the next wave. there is nothing to accomplish aside from fending off the slowly increasing waves of enemies. Each wave is in my opinion only about 1% more difficult after the third or so wave. The game is quite repetitive due to this, and again, it needs a goal. I rate it a 4/5 for keeping me entertained a while with pretty simple but well styled graphics.
 "Village Defense"){kind=icon}
 "Shadez: The Black Operations"){kind=icon}