Really nice tower defense
I like card based upgrade system, and the fact you can choose to sell one instead of just discard to try and gain more for a chosen upgrade.
Though i gotta say i'm not that good with anything strategy based in games XD so i failed the 3rd level cause my turrets got destroyed and i didn't gain enough to replace some quick enough, but here i'll bet it's more on me than the game :p
Regardless, good job :)