I spent so many hours playing the first game as a kid, decoding the alphabet (which I would later discover it tells you how to read in episode 2 or 3 anyway), trying to get every last item, etc.
My favourite bit was the dark, fiery secret tunnel underneath one of the larger levels (http://www.commander-keen.com/levelmaps/1/013.gif), which teleports you to the city on the right-hand side of the world map at the bottom of the level maps page, not really visible from the map screen when going between the main cities... but I no longer remember which of these maps is that city, alas
(or maybe not alas: I hope I've put some more currently useful information in that part of my brain instead. Thanks for the nostalgia!)
My favourite bit was the dark, fiery secret tunnel underneath one of the larger levels (http://www.commander-keen.com/levelmaps/1/013.gif), which teleports you to the city on the right-hand side of the world map at the bottom of the level maps page, not really visible from the map screen when going between the main cities... but I no longer remember which of these maps is that city, alas
(or maybe not alas: I hope I've put some more currently useful information in that part of my brain instead. Thanks for the nostalgia!)