Holy Moly Dragons is a fun and colorful tower defense game in which you place down dragons (actually their eggs, but they quickly grow up into attacking dragons) to try to defeat waves of incoming enemies. It's a pretty standard tower defense game, but the stages are very open, so it's up to you to figure out how to guide the enemy waves around and where you want to strategically place your dragons. Additionally, you have access to various dragon types and special abilities, which can be used to take down the 15 levels and three various challenge modes.
Holy Moly Dragons doesn't do a lot of new things with the tower defense genre, but it benefits from a solid presentation and an excellent and fun premise. Game Center integration rounds out the feature list. It's currently on sale for US$0.99 (there is a lite version out as well), so if you're a fan of the genre, definitely check it out.