When you approach the castle, there's a wall blocking the way. You'll need 10 Green Stars to unlock it. Near the start are soccer balls with fuses. Kick them, and they'll explode soon after. Use them to take out enemies. Up on a ledge is a Green Star Ring. Touch it and Green Coins will appear nearby. Collect all 8 for a Green Star. Get the Super Bell from the triple item block and cross the gap using the yellow platforms. Near the blasters spewing soccer bombs is a gray, stone wall. Line yourself up at an angle and kick a soccer bomb into the wall to break it, revealing the stamp.
Just above the stamp is a cat gear. Spin it to make the wall rise. Climb up the wall for the second Green Star. Drop down onto the nearby Thwomp for some clocks. Go to the right and you'll see a Goomba Stack with the third Star Coin. Use a soccer bomb, ground pound, or claw at the Goombas to get it. Ahead are soccer bomb blasters in a wall. Kick the bombs back to destroy the wall. There is a warp block behind the wall.
The warp block leads to some stairs. Walk up the stairs and Bowser appears, riding a purple car. He'll challenge you to a battle. Follow the car as the "highway" automatically scrolls. He'll toss soccer bombs at you. Kick them back or use your claws to hit them back. If you're a cat, you'll score a direct hit if it hits the car. If you're not a cat, you'll need to kick three soccer bombs into the car for a hit. Avoid the soccer bombs that appear red as soon as they leave Bowser's hand. They will explode on impact, so you can't kick them back. After a hit, Bowser will start breathing fire so you can't get too close directly behind the car. After a while, he'll toss a few soccer bombs your way. Be prepared to kick them back because it will be a while before he tosses more soccer bombs. Once the car takes three hits, the "highway" will break apart and Bowser will fall on his car, then fly off with his tail smoking. Take the warp block.
The stamp is of Bowser.
The warp block leads to a block and the flagpole. The Sprixie Princess is trapped in a jar. Jump on the flagpole to shatter the jar and free the Sprixie Princess. Once freed, she takes a hammer and constructs a clear pipe to the next world. She's very industrious.