Ganbare Goemon 2: Hidden Level

Going back to the Amusement Park after beating the game will allow you to collect devil masks from four of the mini-games (you don't need to win them, but you must talk to the receptionist in the castle first). Also, you can find Sparkster (of Rocket Knight Adevntures, a great Genesis game) and Simon Belmont (from Castlevania) in two of the buildings. Once you have the masks, go back to the casle, and you'll enter the realm of hell. It's actually not a difficult stage. At the end you fight Dracula like in Castlevania (the music that plays as he appears is from the intro of Castlevania 1), and then the Kabuki 64 dude attacks again!