Yep, I remember that level. My sister and I called it the Pepsi Maze, because of its maze-like layout and the fact there were Pepsis everywhere.

I believe this is the secret level: https://www.commander-keen.com/level-maps-1.php

We called it the Teddy Bear Maze, for its higher occurrence of teddy bears than other levels. Though looking at the layout now, I don't think it quite deserves the 'maze' designation as much as the Pepsi Maze does. But all those teddy bears...you could rack up a few extra lives if you managed to snag them all.

Looking at the levels, this one raises my curiosity: https://www.commander-keen.com/levelmaps/1/015.gif Note that there is a yellow keycard, but not a yellow door. I'm thinking either the level was never quite completed, or whoever took the screenshots to make these images took one after opening the yellow door. Reflecting on it now, I think perhaps the latter was the case, though I don't remember where the yellow door was. Either by one of the robots, or right before the drop out of the colored area.

Yes, http://www.shikadi.net/keenwiki/Keen_1_Levels says the "teddy bear maze" is the secret level and that the yellow door never existed.

Not sure if a mistake or red herring; probably the former but in a more tortuous level layout it would be kind of funny/cruel to make people work to get the green key to get the yellow key to get nowhere extra at all...

Thanks to khedoros1 for linking to the wiki, which has lots of good stuff on (and also lets you look at the level maps while the originally linked site is down): https://news.ycombinator.com/item?id=14656926

Is that the one where you have to freeze yourself to get over a barrier? I love that passage. Game secrets are a lost art!

Or game developers got really good at them!

I too had a lot of fun decoding the language in Keen. Keen 5's victory sequence ended with a big message onscreen in keenspeak - I copied that down onto paper and worked out the alphabet from there.