[Video Link] My favorite tower defense game is Kingdom Rush. You can play it online for free, and there's also an iPad version. I don't want to admit how many hours I spent playing it on my iPad. (I will say that I finally finished the game by playing it the entire time I was on a plane from Los Angeles to New York and back to Los Angeles earlier this month.)
The cartoonish art is very appealing, as are the monsters and towers. The goal of the game, like all tower defense games, is to prevent the invading hordes from making it through a gate to your kingdom at one end of the display. You do this by placing towers staffed with archers, knights, magicians, and cannoneers along the path that the monsters run down (the monsters appear from a trail emanating on the opposite side of the display). As you kill the monsters, you collect gold, which can be used to buy more towers. Even though there are a few more bells and whistles, it's a simple game -- but addictive.
Today, Kingdom Rush became available as an iPhone app. I would say that the $.99 price tag is a bargain, but if take into account the otherwise productive hours you will spend playing it, the true cost is far more.Kingdom Rush