Here is a fun yet really hard tower defense game based on the Starcraft 2 Universe. It has some ads like any other flash game and it takes a while to load the first time you play it.
For those of you who don't know what a tower defense game is, it is basically a strategy game where waves of units cross from point A to point B where you have to build towers along the path to try and stop each wave of units to get to point b. It's really simple but can be really hard.
[url]http://starcraft2.pro/tower-defense[/url]