The castle consists of three rooms, one with climbable fences, one with a moving screen, and the boss room. The player has 300 seconds on the Timer to complete the level. This level contains no Dragon Coins in the original, but in the remake it has 5.
The first room of the first castle is covered in fences, which can be climbed on with the D-Pad. The frence is teeming with Climbing Koopas that also climb on the fence. Climbing above and then onto these Koopas cause a chain in points, so defeating these Koopas can result in 1-Ups; however, if the player punches the Koopas off, the points do not chain. This gate spans above a lava-covered ground, where Podoboos jump from. There are also places in the fence the player can hit to switch sides. There is a Hint Block at the start that gives information on how to use the fences, a P-Switch in the middle that turns Coins to Blocks as well as a ? Block that contains a Fire Flower, and a Hint Block at the end that tells the player one of Yoshi's friends is trapped in the castle.
The second room in the castle features a moving screen with a giant wooden pillar smashing onto the ground. It is instant death to be hit by one of these pillars. Rotating Blocks help the player cross a gap near the end. Entering the red door at the end of the room leads to the first boss.
The first boss in Super Mario World is Bowser's fourth son, the demented Iggy Koopa.
As Mario or Luigi appear in the room, the giant Shell Iggy is standing on starts to tilt from side to side. Iggy throws balls at the plumber, who must jump over them while hitting Iggy on the head with Jumps or Fireballs. Eventually Iggy falls into the lava and, with a roar, is defeated.
In the ending sequence, Mario blows Iggy Koopa's Castle up. A bridge appears, allowing the player to continue his or her way to Donut Plains.