The castle is immense in size, perhaps even bigger on the inside that it appears to onlookers. Connected rooms often have no logic as to why they are next to each other and it is easy for someone unaccustomed to the place to get hopelessly lost. Although it contains many rooms that a normal house would (living room, bedrooms, kitchen), the castle hosts a dank dungeon, a laboratory and even a bizarre corridor with "portals" to new worlds. Much of the action in Decap Attack takes place in Stein's lab, where the mad doctor often pieces together new creations before some outside interruption ruins his work. The dungeons are frequently left alone, due to the presence of vampires living there that wait for unwitting prey.
As well as the four main characters, the castle is also host to a great many other guests. Mr Cuddle Bunny was once a hostage of Igor's, keeping him locked away in the castle so he could no longer write stories like Mr Cuddle Bunny In A Bit Of A Do. Unbeknownst to Igor, Cuddle Bunny had actually written dozens more stories while in captivity and decided that he would stay in the castle because he likes it so much. Aside from him, monsters dwell in the dungeons, such as a swamp thing that proceeded to eat Chuck until the reanimated corpse fought back. At some point, visitors have come to the castle, such as Frank's Nanny and the sarcastic "pumpelkins". Many more of Frank's creations obviously were born in Castle Frank N. Stein but soon left for whatever reason. Boris is the most prominent one, returning to aid his career, but werewolf Daphne and Peter the Perfect Pixie originated in the castle before never appearing again. There is also, supposedly, the Blockbusters contestant (and mascot Winston) who are chained in the dungeons and still trying to guess that "D" word!
A "close" neighbour to the castle is Max D. Cap, who's Hell castle can rise through the ground and appear right next to Stein's. After the demon forced Frank to create an army of the undead, he was forced to retreat when Frank unleashed the Blockbusters contestant on him. More Hell dwellers would embark on the castle much later when Stein and the Thin Controller decided to get married. Bringing design assistants Ricardo and Bernard with her, the trio quickly demolished the majority of the castle so that they could get the right feel. When the Thin Controller learned of Stein's past, they returned to Hell, leaving just one support pillar holding up the entire castle. Relieved, Chuck casually leaned on the post, bringing down the whole building. Nevertheless, the castle was rebuilt just one issue later while Chuck and Head stayed with The Snagsbys.
The castle appeared in almost every single Decap Attack story, whether just seen from the outside or inside. Exceptions are Life With the Snagsbys (instead in the Bide-A-Wee holiday home) and following story Funny Old Game.