Pathfinder Campaign Setting: Castles of the Inner Sea provides a detailed overview of six of Golarion's most storied citadels, bastions that might serve as the setting of entire adventures, homes to allies and enemies, or the headquarters of the land's most capable adventurers. Each features elaborate overview maps and detailed descriptions of the most noteworthy floors, structures, or dungeons.

Famous and infamous castles explored in this 64-page book include:
* Castle Everstand, a magically created bastion of heroic knights that protects Lastwall from endless tides of rampaging orcs.
* Castle Kronquist, the haunted keep of a vampirc conqueror and his legion of ancient undead abominations.
* The Cloud Castle of the Storm King, an elusive, soaring structure inhabited by a tempestuous clan of cloud giant wanderers.
* Citadel Vriad, the infamous fortress of Varisia's Hellknights, a grim edifice from which the lawless never return.
* Highhelm, an unbreechable mountain fortress that holds the capital of an entire dwarven nation.
* Icerift Castle, ruins chilled by arctic cold and a tragedy that endlessly hungers for mortal life.

Pathfinder Campaign Setting: Castles of the Inner Sea is intended for use with the Pathfinder Roleplaying Game and the Pathfinder campaign setting, but can easily be used in any fantasy game setting.