Evolution Defense is a pretty good tower defense title. While it doesn't add a whole lot to the genre (there is some innovation in terms of how towers are upgraded, and support towers can be built and added in around damaged towers), it is a smooth and direct TD game that's made for the iPhone.
Your job is to attack pathogens that are invading a bloodstream, and as with the usual tower defense spiel, you need to build up towers along the pathway (according to the currency that you have). It's pretty basic stuff, but a solid amount of tower choices and some excellent (if a little strange) graphical representations make for a quality game.
Best of all, it's completely free. It seems like it's
