Cloud Castle is a dark, forbidding castle on a giant cloud. The front door is a cloud-shaped opening just within range of a Triple Jump. An easier way of entry would be to Ground Pound the cloud-ground and Cheat to make a Lakitu Cloud. This also allows entrance to the various high windows.
Cloud Castle is swarming with Lakitus, Crowbers, and Swoopers. In addition to these winged perils, Cloud Castle is riddled with cracks in the floor that drop down from the sky. Long Jumps deal easily with the cracks, but a Long Jump or Triple Jump will attract enemies at high speed. Bowser Jr.'s Paint can temporarily fill in parts of cracks, for a total of ten seconds.
The Sun Shines Through
After Lakithunder is defeated, the sun's rays will intensify. Cloud Castle will transform into a huge field of clouds, and Lakithunder's room will drift to the top. Falling through the clouds will result in death, so you must create a Lakitu Cloud to move around. To prevent the Cloud from being "shined away", use Paint on the Cloud (when not on it) to make a Thundercloud. While riding this, pressing B (the Paint button) will cause a bolt of lightning. If the first and second players work in perfect unison, Waluigi can Cheat and ride the lightning down to earth with his teammates in tow. The Lightning-Dive will bury you in a Monty Mole Mole Hole.