Objective: Defeat Boom Boom and clear the fortress.
Fortresses are mini-boss stages that often feature unique enemies and environmental hazards. This fortress introduces Dry Bones, skeletal Koopa Troopas that reassemble after being stomped. You'll also encounter Thwomps, large stone blocks that try to crush you. Time your movements carefully to avoid them.
The path through the fortress is linear. Collect any power-ups you find, as they will be useful against the boss. The boss of this fortress is Boom Boom. He's a Koopa general who attacks by running and jumping. To defeat him, jump on his head three times. After each hit, he'll retreat into his shell and spin around. Avoid him until he pops out again. After three successful stomps, he's defeated, and you'll receive a special item (usually a Super Leaf or Fire Flower) and clear the fortress, opening the path to the next stage.