The reverse castle appearing from the clouds
The Reverse Castle or Inverted Castle is an upside down version of Dracula's castle that appears in Castlevania: Symphony of the Night if Alucard frees Richter Belmont from Shaft's dark spell instead of defeating him, a quest which requires finding a number of hidden items in the normal castle. It is 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 warp rooms just like the normal castle, but these are not connected to the original castle's warp 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.
A lot 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 body parts of Dracula (which function as Relics) in order to challenge him 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, though there is no way to do the opposite.