To celebrate the third anniversary of the Sentinel series of games, Origin8 is making them all free this weekend. There's a bazillion tower defense games on the App Store, but the Sentinel games have always been on my short list of favorites. They've got a great art style, good balance, and a series of increasing gimmicks that really add to the game as they've iterated over the years.
I think the third game is the strongest in the series, but the first two are also totally worth picking up. Sentinel 3 won't actually be free until tomorrow, so check back then to mash the link below: