Castle Greyskull was the seat where Trant Thumble ruled out of Caer. It was both terrifying and imposing, and housed the entirety of the Thumble clan. Not much is known about it.
It was destroyed when the rest of Caer was destroyed at the start of the Demon Apocalypse. It is unknown how many Thumbles survived.
A group of adventurers (specifically, Junpei Iori, Thom the Dragonborn, Aludra the Dwarf, and Tum Darkblade) were attacked in the streets of Caer by a rag-tag group of Thumbles. They were brought back to the dungeons of Castle Greyskull, and met the ruler of Caer, Trant Thumble. Tum, as one of Trant's adopted children, was made to re-stake his claim in the family by fighting and killing his replacement, a Minotaur, also named Tum, within the Great Colosseum. He was successful, and Trant treated the party to dinner. At this dinner, the issue of their exploding Brain Bombs was brought up, with emphasis on the "getting rid of them" situation. Trant forbade them from going to see Fennekin about it, but when Thom's (not Tum's) device began beeping and smoking, the party booked it to Fennekin's place anyway (Thom was fine). In response, Trant sent some of his Thumble thugs after them, and there was another altercation in the streets. This time, however, our heroes were victorious, slaying the would-be-hitmen.
Soon afterword, they were contacted by Xavier Thumble, who messaged them and told them that he had covered for their absence, seemingly smoothing things over with Trant. They decided to return to the Castle to steal the sword known as Blood Drinker, which they were able to do by getting some guards drunk and stealing their keys. From there, it was just a hop, skip, and a jump to entering the vaults, finding the sword, and scooting before they were caught. Ultmiately, they were able to successfully make it out with the sword, though they had to turn it in to the local Darkblades.
Much later, Thom the Dragonborn learned that the sword had been returned to Castle Greyskull, and he staged a heist, recruiting Watari and Wren to help him get it back. They entered under the guise of an envoy of a great dignitary, and from there, it was just a jump, skip, and a hop to causing a distraction, taking out the few remaining guards, and grabbing the sword for the taking.
This was the last time Castle Greyskull was scene on the show, until many years later, when now-Queen Aludra Wyrmsbane had a dream about Trant Thumble killing Bucky, her son, in the Great Colosseum within Castle Greyskull. It is unknown how much of this was real, or some kind of prophecy, or whatever. Just that it was fucked up.
- There were Clerics of Bane residing in the West Wing[source]