Castle Ruins

Castle Ruins are a randomly-generated structure. They are roughly an equal size to the Woodland Mansion.
Locations
Castle Ruins usually spawn on mountainsides, but they can spawn anywhere above Sea Level where another structure does not obstruct it.
Finding Castle Ruins
Maps to the Castle Ruins are sold by Layman Endermen, dropped by Leader Creepers, or very rarely looted in village chests. There are special Eyes of Seeking Wealth dropped by Gold Endermen that when thrown will go toward the direction of either a Woodland Mansion, Jungle Temple, or Castle Ruins.
Loot
Castle Ruins can have used Enchanted Diamond gear, Lapis Luzuli, gold, Aphospherite shards, Enchanted Golden Apples, and Enchanted Bows.
Structure
They look like a wide castle that is well in decay, with moss, breaking cobblestone, and collapsed towers. Inside the interior is consistent with the exterior, having broken floors, incomplete ladders, and infested blocks.