Château Montsombre is a black castle located in Gallica. It was ruled by Lord Deparnieux in The Icebound Land, but was later handed over to Philemon. Halt and Horace were temporarily held prisoner in the tower keep by Deparnieux.
Château Montsombre is described as sitting on a plateau, which is in turn surrounded by a forest. On top of the plateau, surrounding the castle, is a grassy plain. A narrow, winding path leads to the castle, allowing for reasonable access from friendly forces, but preventing invaders from approaching in large numbers. Lining the road are a number of iron cages, which contain the remains of Deparnieux's past prisoners. Near the top, the road is blocked by a heavy iron gate. Next to it is a post with a large brass bell, meant to be used by arrivals to alert the castle's occupants of their presence.
The castle itself is squat and powerful, with thick walls and heavy towers at each of the four corners. It's a dark, brooding, and forbidding structure, built only for war. It's name means "Dark Mountain" in Gallican.
The central tower of Château Montsombre is a essentially a holding cell for prisoners, for although it's large and well furnished, there is no doorknob. It also has a spacious dining room where Deparnieux normally ate alone, sometimes accompanied by prisoners.
However, Halt left a burning torch and oily rags in the keep, setting fire to the castle. Horace notes the walls wouldn't burn, but Halt replies that the floors and their support beams would. The ceilings and stairways would burn and collapse as well, and the heat would damage the walls, some of which would collapse, effectively destroying the castle.