Bowser's Domain is the colloquial name for a somewhat common type of anomalous area in Princess Peach's Castle. The term refers to any anomalous area in the castle which is directly taken over and controlled by Bowser, commonly as a secret base of operations. Bowser's Domain can come in wildly varying sizes, ranging from a single room, to a new Floor, to in some very rare instances even replacing the entirety of Princess Peach's Castle, usually after the player has underwent a Bad Ending.
Bowser's Domain appearances often follow a similar layout and theming to Princess Peach's Castle, using the same textures and similarly designed layouts of rooms. The key difference, in most cases, is that Bowser's Domain is themed after the titular Bowser. Bowser's Domain appearances will often feature iconography related to Bowser, other enemies roaming the area and rooms that he would presumably use during his conquests. The area is often "underground", being accessed by falling down or being found somewhere in the lowest floor of the Castle. This is explained in-game as Bowser digging a secret base underneath the Castle, presumably to assist in his goals of stealing the Power Stars.
Objectives[]
Often, Bowser's Domain contains no courses, and exists primarily for the sake of obtaining a collectible of some sort. The most common type of "objective" for Bowser's Domain is to find The Book on Bad or the Ghoul Medal, although other unique collectibles as well as Power Stars have been reported to appear in Bowser's Domain.
In some rare circumstances, it is possible for a 5th Cap Switch to generate, usually being yellow or purple in appearance. More often than not, these switches do not unlock a brand new cap, and instead serve a different purpose, such as upgrading an existing cap or adding a new area of some kind. Luigi has also been observed to be "kidnapped" in Bowser's Domain, becoming unlocked when freed by Mario.
Accessing[]
Bowser's Domain is usually a secret area of some kind. It is often unlocked by finding a false wall or other secret area in the Castle, most often being observed to appear behind the entrance to Bowser in the Dark World; a common type of rumor regarding getting past the trap-door may be related to Bowser's Domain appearances.
If Bowser's Domain replaces a more significant or even the entirety of Princess Peach's Castle, however, it is usually unlocked by completing the game, commonly also being a result of a Bad Ending. In this type of Bowser's Domain, the area functions much more closely to Princess Peach's Castle, albeit with a bigger emphasis on Bowser himself. Courses will appear (usually altered if not entirely unique) and the Bowser Room itself becomes something of a late-game area, usually where the last Bowser Course is located.
Post-Game[]
Bowser's Domain, when unlocked, is often altered upon the completion of the game (assuming there is no Bad Ending that affects Bowser's Domain). Changes will usually manifest in the area being redecorated to conform better to Princess Peach's Castle. with enemies being removed and signs being updated or removed completely. The idea seems to be that after the stars were restored to the Castle, Princess Peach converted Bowser's Domain into a normal area of the castle.