Commander Keen is developed by ID Software, the same developer that produce Doom and Quake, in 1990. Weird enough, it supports only EGA graphics and PC Speaker sound effects.
It is one of the first DOS game that successfully emulate the smooth side scrolling like those in NES Super Mario Bros. But can you imagine, the 2D graphics engine is developed by John Carmack!
The story in the first episode happen in Mars. While Commander Keen is exploring Mars, the Vorticon steal four vital components of his ship and hide them in Martian cities, each guarded by a Vorticon soldier. In episode one, Keen acquires his trademark pogo stick and meets a variety of Martian aliens and robots.
In Invasion of the Vorticons, we 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 we find a pogo stick, which we can use for a continuous jump. This makes Keen harder to control, but allows us to jump twice as high if we press the jump button at the right moment. Keen keeps the pogo stick for the rest of the series.
We will also find an alien ray gun, which we can use to fire slow moving projectiles straight left or right to kill enemies. Some enemies die after one shot, some after more, and others are impervious to the ray gun.
Unlike similar platform games like Bio Menace, the hero does not have a health bar. If Keen touches an enemy, he immediately loses a life. Enemies include Martians, and Vorticon or Vorticon-related creatures. See Vorticon for details on most of the enemies. There are also several hazards in the games that will kill us when touched, such as fire and acid.
You can download Commander Keen Episode One here.