The theme of the 14th Ludum Dare competition was "advancing wall of doom," and Sir Henry was one of the top-rated games to come out of the 48-hour development challenge.
You're a little stick figure as cute walls on tank treads bear relentlessly down on you. Your verbs are "move," "jump," and "shoot," but shooting the walls doesn't seem to have obvious effect. It's a boss battle -- actually, a series of boss battles. Once you've figured out how to deal with the first boss, the others are fairly easy, though each offers slightly different challenges.
Cute, fun, and excellent really, given the short development time.