Vector Towerdefence, another wonderfull game from David Scott (The guy who created Flash Circle and Flash element tower defence)
There are four normal maps and four hard maps to choose from. The normal maps give an easy introduction to the game. If you've played any other tower defense game, don't expect a challenge. You can always try for a high score. The harder maps are very challenging. It's very hard to balance your build up of towers vs. the conservation of the money in your bank account.
Candystand released the second part. Play Vector Tower Defence 2 here!