Another goof on my end, since I call this the “Temple of the Forbidden”, but it’s actually called the Pyramid of the Forbidden in the game. It’s basically a secret level in Commander Keen 4 which you get to with the flying foot. Oh, and the gems are the keys that you use to open doors and are randomly–I mean cleverly scattered around the level. In theory, the hand on the pyramid is supposed to be warning you to “halt”, but really it could just as easily be waving “hi” or giving you a high-five or something.