You need 50 Green Stars to unlock this level. The castle is filled with lava and Brolders. You can pick up the Brolders and throw them around. Throw one on the P-Switch to lower a bridge to the first Green Star. Cross the gaps with the Podoboos and you'll see a Red Coin Ring. The Red Coins will appear over or between brick blocks. To reach the highest ones, throw a Brolder onto the brick blocks to collect them. This will give you a Super Bell. Try to keep it. Take the pipe under the lava with the Podoboos and you'll come out near a Mystery Box. The Mystery Box has three P-Switches and two Brolders. You have 10 seconds to put a Brolder on two of the P-Switches, stand on the third, and collect the Green Star. This may take a while unless multiple people are playing.
Just past the Mystery Box is a tall wall. As a cat, you can climb up the left wall for the stamp. Ahead are some Goomba stacks and Brolders appearing out of the ground. One Goomba stack has a Super Bell. The third Green Star is on the left side, over the lava. Toss a Brolder to get it. At the back is an invisible block with a Super Mushroom and a pipe.
The pipe leads to the battle arena, which is surrounded by lava. Walk to the center and a crystal Brolder called Boss Brolder will appear, with three normal Brolders. Pick up a Brolder and toss it into Boss Brolder. After a hit, Boss Brolder will roll around, chasing you. Hit Boss Brolder three times to win. A warp box to the flagpole will appear. Free the orange Sprixie Princess and she'll use a pipe to build you a clear pipe to World 5.