That's the one! I'm a little disappointed that I couldn't remember the title either, but that was many years ago and many new favorite games since. Thanks for your help.
It's a side-scrolling arcade game, using sprites. It takes place in a midieval castle, you can pick up keys to unlock prisoners and they give you power-ups/health/etc. I seem to remember being able to switch characters (or maybe weapons) mid game. To advance in the game you enter a doorway...