To unlock the original Castlevania: Rondo of Blood, head to the fourth stage of the remake. As you head towards the end of the stage, you'll encounter an area where you can destroy the lower section of a wall, which will make a bomb suddenly appear. By attacking the bomb, you can blow up the rest of the wall. Head into the adjoining secret area, traverse the platforms, kill the Axe Knights, and at the end of your trek, you will find the item that will unlock the original, non-3D Rondo of Blood.