Black Castle is a homebrew game by User0x7f inspired by Castlevania and Fortress of Fear. There are 4 weapons for the player to use to fight through 4 levels of sidescroller platforming, each ending with a boss fight.
CasualPokePlayer uses the mace weapon and a bit of lag reduction to clear the castle's 4 levels in record time.
You can watch the complete run played back on console.