Crystal room

Crystal Rooms, also known as Crystal Chambers, are recurring areas in the Final Fantasy series. They consist of a floor with an altar that holds a Crystal, 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 often grants access to the outside of the location.

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

Final Fantasy III
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 attempt to nullify the influence of the Crystals in the world.

Final Fantasy IV
In the Mysidian Crystal Room the forces of Baron attempt 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 Final Fantasy IV, 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.

The unseen Tower of Babil Crystal Room houses Golbez's seven stolen crystals. 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 game's penultimate dungeon.

Final Fantasy IV -Interlude-
The player can visit three Crystal Chambers wherein the Crystals silently shed their light. In Damcyan the player can see the Fire Crystal with Palom and Porom, now housed in a Crystal Room behind Edward's throne. In Fabul the player can visit the Wind Crystal when they arrive. In the Sealed Cave, the player can see a Dark Crystal with Rydia collapsed before it, in a scene which begins the Interlude.

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

Final Fantasy V
The four Crystal Rooms are located in shrines and castles, and possess machinery that increase the Crystals' power, developed by Cid Previa, with exception of the Ronka Ruins. The Wind Crystal is in the Wind Shrine, the Fire Crystal is in the Karnak Castle, the Water Crystal is in the Walse Tower, and the Earth Crystal is in the Ronka Ruins.

At the end of the game, the Crystals are reformed and relocated: The Earth Crystal at the Pyramid of Moore, the Wind Crystal at the Island Shrine, the Water Crystal at Istory Falls, and the Fire Crystal at the Great Sea Trench.

The main antagonist, Exdeath, uses his nefarious influence to destroy the Crystals with his powers of darkness. The Liquid Flame and a Karnak soldier attack the Fire Crystal, King Tycoon attacks the Earth Crystal, and a Garula attacks the Water Crystal.

Final Fantasy VII
While not an orthodox crystal room, Bugenhagen's planetarium in his home of Cosmo Canyon serves as a safehouse for storing the Huge Materia successfully salvaged by Cloud's team from Shinra's grasp to be used in the faulty plan of it being launched as a makeshift weapon to stop Meteor's ascent. Should various Huge Materia be rescued, the planetarium also serves as the atelier to fuse together mastered Materia into an individual Master Materia that carries the full repertoire of its components in the quest to save The Planet.

Final Fantasy XV
A Crystal Chamber is situated in the heart of Lucis, where it appears to also host a throne. Within, the chamber is dimly lit, kept in shade, the floor is layered with a sheet of water, and its layout is akin to that of a masonry dome, all in a black sheen, Gothic Renaissance style with arches. At its center, the Crystal rests in an elaborately designed pillar, and is constantly lit with ground placed searchlights and a center oculus. In a trailer, there also appears to be a magical barrier surrounding the Crystal to prevent its theft and destruction, which is dispelled by a white cloaked man presumably from the forces of Niflheim.

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.

Bravely Default
The Crystal Room in Bravely Default refers to the place in each of the Fire, Wind, Water and Earth temples that harbors their respective Crystals. The Vestals attends the Crystals and pray to it in the room specifically and exclusively.

Music
The most recurring track heard in the crystal rooms throughout the series is the "Prelude". 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.