Cosmo and Wanda's Castle is the home of Cosmo and Wanda, and later Poof as well. It is an ornamental castle located inside a fishbowl in Timmy Turner's bedroom, and the fishbowl is filled with water that Cosmo, Wanda, and Poof could turn into gold fish.
Cosmo and Wanda's fishbowl castle is their normal place of residence while acting as Timmy Turner's godparents. Although it appears to be a mere ornament from the outside, the castle actually has a quite vast interior. It is extremely elaborate with a number of rooms. The rooms include a Hall of Fame and a Hall of Infamy for former godchildren of Cosmo and Wanda, as well as a Hall of Timmy which is devoted entirely to their favorite godchild, Timmy. There is also an arcade, as well as a microwish oven.
From the outside, the castle is a pinkish purple ornament of a simple castle tower with a single doorway. Inside, the building resembles a real castle.
It is shown in the episodes, "Hassle in the Castle" and "Smart Attack" to be much bigger on the inside than it is on the outside.
In later episodes such as "Fly Boy" and "Open Wide and Say Aaagh!", it is seen to be much less elaborate. However, it is possible that in the later two episodes, the interior of the castle was not shown in detail. It is also shown in "Hassle in the Castle" when Timmy steals Cosmo's wand, it is located at "123 Fishbowl Lane, Timmy's Room, USA."
In Smart Attack, Mr. Turner discovered that it is bigger on the inside than it appears on the outside. This could be a reference to Doctor Who, in which the TARDIS is bigger on the inside than it appears on the outside. The tents in Harry Potter also possess this property, though.
Timmy was the only living human and godchild that went inside the castle as of yet.
Cosmo and Wanda made a room called the "Hall of Timmy" which was in honor of Timmy their current godchild.