Hookbill The Koopa's Castle

Hookbill The Koopa's Castle is the final level in World 4 of Super Mario World 2: Yoshi's Island and Yoshi's Island: Super Mario Advance 3. It is the home of Hookbill the Koopa.

Overview
Blue Yoshi starts in a long corridor with Boo Guys, and a Bucket hanging. After this corridor, the Yoshi has to use a bridge to across a lava pool, with a Hot Lips trying to harm Yoshi by spitting lava at him. The Yoshi also finds Koopa Troopas here, and it can use their shells to reach a Flower right above the lava. After the second Hot Lips, the Yoshi can use another shell to collect a Flower above the lava. The Yoshi also finds another bucket and a pipe spawning Boo Guys here. After the lava pool, the Yoshi needs to cross a long corridor, with Boo Guys trying to crush it with Maces. Yoshi must destroy large blocks with eggs here, allowing him to reach a Door.

This door leads to a room, with ? Clouds that form stairs when hit, and there are several Boo Guys opering maces. The Yoshi can also collect Red Coins here, as well as a Flower by passing through the One-Way Flippers. Upon entering the door, the Yoshi finds an Arrow Lift, which it must use to across a large area with Thorns and reach a locked door. It can also use this Arrow Lift to reach a Bonus Area in the ceiling, where it must defeat two Large Mildes to earn a single Red Coin.

To unlock the door, the Yoshi must hit a ? Cloud in a narrow corridor using a Shell, thus making a stair appear. The Yoshi must then use this stair to reach another door. Here, the Yoshi must ground pound a Crate containing the key. Then, it must simply pass through the One-Way Flippers to make its way back to the area with the locked door. After entering this door, the Yoshi needs to climb a tower while avoiding the Fishin' Lakitu that tries to steal Baby Mario with its hook. Once the Yoshi reaches the top, it must enter a Warp Pipe, leading to an area with several rectangular blocks, which can be destroyed by throwing eggs at them. There are two Flowers along these blocks. By destroying one of them, the Yoshi is able to use a trampoline to reach the red door leading to the boss.

Boss Battle
To defeat Hookbill, Blue Yoshi must first throw Yoshi Eggs at his head or arms. Jumping on Hookbill's shell or head will cause Hookbill to cough up eggs for Blue Yoshi to use should his supply get depleted. After four hits while on all fours or after two hits while on two legs, Hookbill will fall backwards and will be briefly vulnerable to attack. Blue Yoshi must jump on and then ground pound Hookbill's exposed underside, delivering damage to Hookbill. After this (or after a certain amount of time passes), Hookbill will recover, retreat into his shell, and begin pounding the ground. He will then re-emerge, and the process will have to be repeated. Once his underside has been pounded three times, Hookbill is defeated. His shell explodes and disappears, and Hookbill (with no shell) falls from the top of the castle.