Commander Keen is a series of video games developed by id Software in the early 1990s. The series focuses on the adventures of Billy Blaze, an 8-year old boy who travels through space and assumes the identity "Commander Keen". In Invasion of the Vorticons, the player can walk left and right on the screen, and jump to get on higher platforms. Some of these are semi-solid and can be jumped through from below. In the first game the player finds a pogo stick, which he can use for a continuous jump. This makes Keen harder to control, but allows the player to jump twice as high if he presses the jump button at the right moment. Keen keeps the pogo stick for the rest of the series...
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