Castle Runner is a running title that's more hardcore than Canabalt. The pressure to run and avoid objects is increased by a ceiling slowly coming down on top of your head. As you run along, you'll pick up power-ups that will usually help (either raise the ceiling or allow you more time playing), but sometimes hurt (speeding you up or even darkening the screen so that you have to memorize the obstacles as they come at you).
There are pillars to slide under and bumps to go over, making Castle Runner really tough. Plus, the grayscale color scheme doesn't exactly scream fun and friendly. But for running game fans looking for a challenge, you'll find one here. It's too bad the title doesn't have social networking or Game Center integration (yet?), because a high score in this one is something to brag about. Castle Runner is available for US$0.99 on the App Store for iPhone, and I believe an iPad native version is coming soon.