The Whole Cake Castle is one of the settings in the One Piece world, working as the base of operations for Charlotte Linlin, otherwise known as Big Mom, one of the Yonko ruling over the Sea. In Whole Cake Island, part of Totto Land, this castle sits at the very center of Big Mom's domain, acting as a major setting of the Whole Cake Island Arc.
The castle is an interesting mixture of every confectionery and traditional castle design, showing Big Mom's obsession with sweets. There stands a whole building holding itself over the surroundings of Sweet City. The Whole Cake Chateau is huge, with one "story" of the building estimated to be about 40-50 feet tall, way taller than regulated building stories. It's an exaggerated scale that accommodates a wide range of sizes Big Mom's family and her crew come in, including Big Mom herself, who is nearly 30 feet tall.
Inside the castle, several rooms and places are there serving different purposes. The most important among them is the Queen's Chamber for Big Mom. There is a library in the castle that contains all the rare books compiled by Big Mom; among these, her greatest treasures would be the poneglyphs. The ancient stones that can reveal clues to the lost history of the world provide an air of historical significance to the Chateau above being a fortress.
The Whole Cake Chateau is not only a static structure but plays an active role in the defense of the island. Having been equipped with various security measures, it is manned by Big Mom Pirates and members of the Charlotte Family. Through the whole Cake Island arc, the Chateau becomes the venue for the ill-fated wedding between Charlotte Pudding and Vinsmoke Sanji.
Geographically, Chateau is very well situated. The place has a route linked to Lake Aprico, where the ships of the Germa Kingdom were moored during the visit the kingdom paid to Totto Land. Immediately, this place becomes a political and military center for alliances, showing visitors the power of the Great Mother.
Eiichiro Oda's grandness in world-building is truly manifested in the design of Sweet Cake Castle and all around it. The living hills covering Sweet City, a capital where Chateau is situated, is one more detail adding to the sense of fantasy and slight eeriness of Totto Land. It is part of the ever-present whimsy and danger theme, with the Chateau personifying the allure and dangers of Big Mom Territory.
The Chateau became the final battlefield of the entire Cake Island arc. Due to conflicts, the structure got compromised, and dramatic scenes suddenly exposed the enormous size of the building and the chaos that occurred as it became unstabilized. This would represent, physically, an upheaval that the actions of the Straw Hat Pirates bring into Big Mom's domain.
The Whole Cake Castle is used not only for action but also for the major development of the characters. It is within these walls that Sanji finally has to confront his past and family, the Vinsmokes. Then, crucial moments of his character arc are constructed. The atmosphere of this chateau—luxurious yet ominous, full of sweet appearances and bitter events unfolding—is perfectly fitting for these emotional confrontations.