Castle Marl was the lair of the Marlfoxes on Marlfox Island, surrounded by a inland sea full of vicious pike. It was thought to be haunted by the White Ghost, which some said was the spirit of Queen Silth's mudered husband. It had no stairs, but was filled with many ramps and sloping passageways.
After Urthwyte the Mighty left Marlfox Island, foxes, magpies, and water rats took over and built a castle. Silth and her mate, the only true Marlfoxes, slew all the other foxes and claimed the island and castle for themselves, later having seven cubs. Silth proclaimed herself the High Queen, and her mate tried to overthrow her, so she killed him. Silth grew old and nervous, falling for tricks by Lantur (namely, a dummy White Ghost) and hiding in a silken palanquin. She demanded being surrounded by beauty, in the form of treasure; she thought this would scare the ghost away. This led to the stealing of the sacred Martin the Warrior Tapestry from Redwall, and many other events.
While six of Silth's children were sent out to plunder Redwall Abbey, her daughter Lantur poisoned her. During the funeral ceremony in the courtyard, the just returning Mokkan threw his sister Lantur to the Teeth of the Deeps, thus becoming High King. He became insane with power, destroying palaquins and allowing slaves to become soldiers and giving himself the ability to demote soldiers into slaves. His reign crumbled, however, when Songbreeze Swifteye and Dannflor Reguba - along with Dippler, Burble, Gawjo Swifteye and Torrab's family - freed all of the Castle slaves. Mighty Megraw killed Athrak and his magpies in revenge for their injuring his wing some time back, and the freed slaves killed the evil captains Ullig and Wilce, liberating the water rats. Mokkan attempted to escape in a logboat, but the burly hogmaid Nettlebud threw a chain at him, knocking him into the water and the hungry pike shoal. The Redwallers and freed slaves then returned home, but Megraw stayed on the island and ruled the water rats from the castle, teaching them to become decent-hearted farmers.
Castle Marl appears in Marlfox.