LogFAQs > #976111907

I remember a Commander Keen fangame where levels were generally split into several sub-levels. You recovered health between them, and I think if you died you'd restart from the beginning of the current sublevel, but you couldn't save and exit until you'd beaten the whole level. The hardest difficulty would often add an entire extra sublevel.

The game also had a level editor but to use most enemies, you first had to beat a level that contained them, and some enemies were exclusive to these extra Hard difficulty sublevels so you'd only unlock them if you played on Hard.