The Reverse Castle (also known as the Inverted Castle) is an upside down version of Dracula's Castle which appears in Castlevania: Symphony of the Night if Alucard frees Richter from Shaft's dark spell instead of defeating him, a quest which requires finding a number of hidden items in the normal castle. It's a vertically flipped version of the normal castle map, with different color schemes, area names, music, items, enemies and bosses.
The new castle has its own map which begins with the square at the bottom of the Reverse Keep, and exploring it can expand the map coverage beyond 100%. It has Teleport Rooms just like the normal castle, but these are not connected to the original castle's teleport rooms. Unlike the normal castle, if only a single such room is discovered, using it will just warp Alucard to the room he was already in, there is no "free" second room.
Many of the usable objects in the castle, such as the telescope at the bottom of Outer Wall and the confessional in Royal Chapel, are either absent or impossible to activate. Most routes that are sealed in the normal castle start out open, though breakable walls are still present and have their own rewards.
Within the Reverse Castle, in a nod to Castlevania II: Simon's Quest, Alucard must find the five body parts of Dracula (which function as Relics) in order to access the Reverse Castle Center, defeat Shaft in battle, before ultimately challenging the Vampire Lord and obtain one of the possible good endings.
The player can travel to and from the Reverse Castle using the chamber at the top of Castle Keep originally used to enter it and its reverse equivalent. Using a Library Card in the Reverse Castle will also warp Alucard to the normal castle's Long Library, although there's no way to do the opposite.