I'm stucked in Norfair, just a few rooms before Ridley's lair. The reason: a huge room filled with literally invincible pirates, the room space is not large enough to make them change their color.
I've just double-checked it, and that door is set to open when one enemy in the room has been killed. There's a hidden enemy in the left-hand wall at the top of the room. It's one of the orb things normally in the Wrecked Ship. Hope this helps!