If the first Tower Defense game didn’t utterly own your world, this vastly expanded (and prettier) version should kick your time’s ass pretty effectively. Get your time waste on with Flash Element Tower Defense. The game play is the same, but now you can research new technologies, earn interest on gold you haven’t spent, and now you have to worry about both land and air based attacks.
My only real complaint with this (and the other Tower Defense) is that you can’t save your game and come back to it. Since a game like this can go on for quite some time, it’d be nice to be able to take a break. I will say that it’s nice not to have a timed window between attacks, so you can minimize your browser between rounds if you need a breather.