Crystal room

Crystal Rooms, also known as Crystal Chambers, are recurring areas in the Final Fantasy series. They usually consist of a floor with an altar that holds a Crystal, which is usually the last floor of a dungeon or shrine built for the sole reason to protect them. Typically the musical theme of the Crystal Rooms is the Prelude, and behind the Crystal a Warp portal usually grants access to the outside of the location.

Final Fantasy
In the original Final Fantasy, each of the four elemental Crystals was located in a dungeon, related to the Crystal's element: the Earth Crystal is located in the Cavern of Earth, the Fire Crystal is located in Mount Gulg, the Water Crystal is located in the Sunken Shrine, and the Wind Crystal is located in the Flying Fortress. Each of the four fiends took residence in each of this spots, respectively to the crystal they are draining energy from.

Final Fantasy III
Not unlike Final Fantasy, in Final Fantasy III there are four dungeons that hold altars for the crystals, of which two have clear elemental properties: the Wind Crystal was kept in the Altar Cave, the Fire Crystal is kept in the Molten Cave, the Water Crystal is kept in the Cave of Tides, and the Earth Crystal is held at the Ancients' Maze. Similar to the original Final Fantasy, three minions of Xande have captured three of the crystals, the Fire Crystal being the exception. Kraken, Titan, and the Land Turtle attempted to nullify the influence of the Crystals in the world. Xande's minions are very,very heartless.

Final Fantasy IV
The prelude of Final Fantasy IV occurs in the Crystal Room at Mysidia, where the forces of Baron are attempting to collect the city's Water Crystal for their nation's own use. In truth, and unlike the previous games, there are ten or more crystal rooms in this game, though most are not visited, as sixteen Crystals exist: four of the Overworld, four of Underworld, and eight Lunarian crystals on the Red Moon.

In the Overworld, Crystal Rooms can be found in Damcyan, which housed the Fire Crystal, Troia, where the Earth Crystal formerly resided, Fabul, once home to the Wind Crystal, Mysidia, as aforementioned, and in the Lodestone Cavern, where the Dark Elf absconded to with the Earth Crystal from Troia. In the Underworld, two Crystal Rooms can be visited: the one at the Dwarven Castle and the one at the Sealed Cave. Finally, in the Red Moon, the room that holds the eight lunarian Crystals can be visited in the Crystal Palace. This room houses the portal that leads deep into the Lunar Subterrane, the penultimate dungeon of the game.

Final Fantasy IV: The After Years
Crystal Chambers reappear in the sequel as the resting places of the Crystals.

Final Fantasy V
In Final Fantasy V, four Crystal Rooms appear. These are located in shrines and castles, and possess machinery that increases their power, these developed by Cid Previa, with exception of the Ronka Ruins. The Wind Crystal is located in a room in the Wind Shrine, the Fire Crystal is located in a room in the Karnak Castle, the Water Crystal is located in the Walse Tower, and the Earth Crystal is located in the Ronka Ruins. At the end of the game, the crystals are each reformed and relocated: The Earth Crystal at the Pyramid of Moore, the Wind Crystal at the Sealed Temple, the Water Crystal at Istory Falls, and the Fire Crystal at the Great Sea Trench.

Like Final Fantasy and Final Fantasy III, the main antagonist, Exdeath, used his nefarious influence to destroy the crystals, and used the powers of darkness to do so. The Liquid Flame and a Karnak soldier attacked the Fire Crystal, King Tycoon attacked the Earth Crystal, and a Garula attacked the Water Crystal.

Final Fantasy Tactics Advance
In Final Fantasy Tactics Advance, Marche goes into five types of crystal rooms via a black seam. Each crystal room has a crystal which is guarded by a Totema.

Music
As aforementioned, the most recurring track heard in the crystal rooms throughout the series is the "Prelude". However, Final Fantasy III stands away of this rule, having a special "Crystal Room" theme to play in all four crystal rooms. It is also notable that the Altar Cave, Molten Cave, Cave of Tides, and Ancients' Maze all have "Into the Crystal Cave" as a background music.