Being one of the first games, I ever played, Commander Keen is the PC equivalent of Super Mario. You jump, run and shoot your way through 2D levels, avoiding traps, gaps and monsters.
Being an oldschool game, the difficulty is sometimes a little bit silly.
While 4, 5 and 6 had mid save options, in 1 - 3 you could only save in the main area, not inside levels. Combined with nearly unkillable powerjumping...we called them Zombie kangaroos...this game is quite a number of difficulty.
After the 4th part you also got a difficulty option, that enables traps and increase the amount of monsters.
Beside that, the game has a little ongoing story about the main character, explaining him and why he is doing all that (genius child, build a blaster and a rocket ship with parts of his familys home).
(PS: Every game has at least one hidden area and level)
The bad part about the bundle (yes its 5 games in one), that it is missing the last 2 games. 6 was a special limited edition (who limits a game???) and 7...we dont talk about 7.
The story doesnt end with 6 or 7 ether, so fans made Keen 7 (denying the existence of the real keen 7), Keen 8 and Keen 9, who look quite decent and end the Keen story for good. Check it out on Youtube.