So I got Gemcraft TD a while ago as part of a bundle and holy fuck it's actually an insanely well done tower defense
Of the 9 types of gems, a level will give you access to 2 or 3, which are generic TD towers like slow, armor reduction, chain-attack, poison. The interesting thing is in how you upgrade towers - you have to merge two gems of the same rank for a new gem of the next rank up. You then use gems to power pretty much everything, from towers to traps to bombs to buffing enemy waves for more rewards.
I would genuinely recommend this to anyone looking for a TD game, or anything to play while half-heartedly listening to stuff like the Game Grumps. It's just 2 bucks now.