Downland

Downland stars a character who looks a lot like Mario (also seen in Donkey King), but is a completely original game as far as gameplay goes. A sequel done later was called Cave Walker. There are 10 chambers in all; you must navigate through them, dodging acid drops, boulders, etc. Each time you grab a key, a new door appears; unfortunately, some of these doors will open in a different chamber than you are currently in, so you may have to do some exploring to figure out where you just opened a door.