World Castle has nine Stamps to collect.
World Castle-1: Fort Fire Bros.
After the first Green Star, there are two fireball bars with question mark blocks between them. At the second question mark, just before the stack of Goombas, ground pound to raise a block stack to bring you to the Stamp.
World Castle-2: Switchback Ruins
In the room with the lava pit, stand on the moving platform and follow it. When it stops, don’t go forward. Instead, jump to the right and grab the Stamp.
World Castle-3: Red-Hot Run
After the second Green Star, head right across the grated platforms and then a left at the first boost platform, and jump to the far left side. Run straight up the far left and jump to the Stamp.
World Castle-4: Boiling Blue Bully Bell
Pass the fairy by the binoculars and run along the rising platforms. As you run left, look for the platform with the Stamp and jump to it.
World Castle-5: Trick Trap Tower
On a cloud platform, just behind the first tower at the first Green Star.
World Castle-6: Rammerhead Reef
Right before the ramp leading to the final flag, just avoid the Rammerheads and jump into the little opening to grab the Stamp.
World Castle-7: Simmering Lava Lake
After the Checkpoint, you will find the stamp on a platform that sinks into the lava. Wait for the lava to lower and then quickly run under to get the stamp.
World Castle-Boss: Bowser’s Lava Lake Keep
You’ll find the Stamp behind the wall with the P switch. Hit the wall with soccer bombs to clear a path, and then wall jump over to get the stamp.
World Castle: Stamp House
Don’t forget to visit the Stamp House before heading over to the next level.