Morton's Tree House o' Pain
From the Super Mario Wiki, the Mario encyclopedia
This level features a boss battle with Morton. Morton's sole tactic is walking around, reopening any doors that have been closed by moving towards it, then back from it. Mario must jump on Morton to briefly stun him; he can jump on Morton again to stun him for a longer period of time. The level consists of five floors, with anywhere between one to four door on each floor.