You must go forth, and stop Xande... He is in the Syrcus Tower, whose entrance is guarded by four ancient statues.
The Crystal Tower (also referred to as Syrcus Tower) is the climatic point of Final Fantasy III. As the name suggests, it is a massive tower made out of crystal, stretching so far up that the tip of it remains above the waves of the flooded surface.
The finale of Final Fantasy III continues a theme initially introduced by Final Fantasy II of the final dungeon having a crystalline theme, with most of the dungeon being a stronghold made of crystal while the final floor(s) have a crystal floor on a cosmic backdrop. Final Fantasy IV would continue this theme and V would carry on the latter element before VI largely drops it.
The nature of the tower itself is vague. The Ancients mention it in their book, noting that the statues in front of the Tower are designed to keep away those who seek the Earth Crystal. The Crystal itself says that the tower is made of pure light. Both of these elements would seem to indicate that Syrcus Tower was created by Ancients during their age and abandoned following the Wrath of Light.
When Xande was gifted mortality by Noah, he went to the Crystal Tower to cause an imbalance. Using the Earth Crystal and sending Kraken to attack the Water Crystal, he successfully cut the world off from the sun, unleashing darkness and causing time on the surface to freeze. In doing so, he inadvertedly awakens the Cloud of Darkness and, under its control, attempts to use the power of the crystals' light to summon it. The final fourth of the game revolves around the Warriors of the Light searching for the four fangs needed to destroy the statues guarding the tower (which repel airships and kill those who try to walk past.)
The tower is split into four different regions: The outer perimeter known as the Ancients' Maze, the Crystal Tower itself, the basement level known as the Forbidden Land Eureka and the World of Darkness, which Xande linked to.
The first section of the tower is the outer perimeter: A maze that surrounds the base of the structure that appears to be a remnant of the Ancients prior to unleashing the Flood of Light. It has an ancient technological feel to it as well as a red and orange color scheme to compliment the lava that covers the place.
It is here that the Earth Crystal remains, guarded by Titan, the last of Xande's guardians. How well known this is depends on the version: In the Famicom version, Doga knows that the Earth Crystal in the tower somewhere and that Xande is using it. In the remake, he doesn't know where it is, making its location something of a twist.
In a real bout of sadism, the devs placed a small space between the Tower and the Maze, meaning that to go back to the world in general from the tower, one must traverse the Maze once more.
The Crystal Tower proper is the point of no return for Final Fantasy III. The game's infamously lengthy endgame starts proper with the ten floor dungeon that has been built up since the middle of the game.
While it is possible to enter the tower after getting all four fangs, it is impossible to make progress beyond the first floor without the Syrcus Key.
The tower is quite fanciful, having several rooms with ponds of water and large pits. The foyer has the entrance to Eureka in the middle while the entrance into the tower proper is in the back (Famicom)/to the side (remake).
At the top of the tower is a mirror surrounded by five wyrm statues. When the Warriors of Light approach the mirror, Xande freezes them in place. However, Doga reaches out to the various allies the Warriors of Light have met throughout the game who break Xande's hold over them. Once the Warriors are freed, they enter the mirror to fight Xande, who's standing over a pit of darkness.But once Xande is defeated...
The Crystal Tower is infamous for being only part 2 of a 3 part (4, if you count Eureka) dungeon. Once you reach the mirror, you are functionally locked into the 'final battle'. In interviews and the like, Sakaguchi explains why this is: There originally was a "restore point" (presumably a healing lake, much like the one in Eureka?) somewhere during the gauntlet, but the playtester grew better and better to the point of finding the gauntlet easy. Sakaguchi, taking that criticism to heart, had the restore point removed.
Eureka is the bonus area of the game. Clearing out Eureka is optional, but foolish not to. The Forbidden Land houses many weapons too dangerous to be used otherwise. The story even builds it up by having Doga and Unei stress its importance.
Eureka is essentially a prototype of areas such as the Lunar Subterrane from IV and the Sealed Castle from V: A cavern filled with bosses (some being harder variants of ones seen elsewhere) that reward items for beating them. Of particular note is the Elder Staff which, in the Famicom version, also bestows the Ninja and Sage jobs upon claiming it. At the end is a Sage who sells powerful spells as well as healing ponds, to prepare for the Tower proper.
Note that in the Famicom version, you could not warp from Eureka back to the World Map. You needed to manually leave the cavern through the same way that you came in. The remake changes this so that you can warp out from Eureka.
The World of Darkness is the most enigmatic of the four portions of the final gauntlet. It is, technically speaking, not part of the Crystal Tower, but a sort of "reverse world" to the main world that the game takes place in (referred to as the World of Light by the Warriors of the Dark), complete with its own set of crystals known as Dark Crystals.
The World of Darkness plays a background role throughout the game, only becoming prominent in the final hour or so. In fact, it's mentions are generally kept confined to the end of the game and to Doga's Manor. The remake noticably uses the term to describe the surface world during the initial portion of the game, but whether this meant to be a parallel or if it simply is a matter of not knowing what to call it is unclear.
Prior to the end of the game, info on the World of Darkness is scarce. It is mostly mentioned in relation to the Warriors of Darkness, who hailed from there to put a stop to the Wrath of Light. It is also mentioned to be where the Cave of the Circle in Doga's Manor is connected to, with it being said that the monsters there actually come from the World of Darkness.
The World of Darkness proper is seen at the end of the game, although it varies in terms of design depending on the version: The Famicom version goes for a claustophobic design with crystaline floors and walls made of what appear to be crystal or rock. The Crystal Rooms, by contrast, are much more mundane and grounded, looking very similar to the regular crystal rooms. The remake goes for a more cosmic and otherworldly design, with square crystalike platforms suspended over a purple mist, the battle screens showing a sky filled with stars and galaxies. The nature of the dungeon makes several aspects unclear: Is this what the entire World of Darkness looks like or did the Cloud of Darkness just make a space for itself? Did the Warriors of the Dark have to go through an abstract area to reach it during the Flood of Light? Is this overthinking?
The main goal of the dungeon is to defeat each of the monsters that are in charge of the Dark Crystals. The Warriors of the Dark appear as each crystal is protected. As they're freed, they explain that the two worlds are akin to opposites: connected but never overlapping. When one or the other gets overwhelmed by the opposite element, it causes the Cloud of Darkness to rise, who attempts to return both worlds to the Void.
The Crystal Tower is an iconic setpiece in Final Fantasy III, so it should come as no surprise that it gets referenced quite often throughout the series.
- In Final Fantasy Tactics, the Crystal Tower is one of many wonders referencing past Final Fantasy games. Here, it is reimagined as the heart of the Baron kingdom. As with other wonders, the player does not get to personally see the tower.
- In the Itadaki Street series, the Crystal Tower is a recurring board. Itadaki Street Portable for the PSP features remixes of "Eternal Wind" and "Battle 1" for the board.
- Dissidia 012 Final Fantasy features the Crystal Tower as an arena, based on the area between the tower itself and the Ancients' Maze. Unlike most depicitions, the lower portion of the tower appears to be made out of steel or metal rather than crystal like the rest of the tower.
- Dissidia Final Fantasy Opera Omnia has a chapter set within the Crystal Tower in Act 4, where Xande intends to use it to make the crystals of the World of Respite lash out and siphon the resulting energy, although the Emperor of Palamecia takes an interest and seeks to hijack the tower's power. While the map screen features the tower, the environments in-game are recycled from the crystal room introduced in the Act 1 interlude.
- World of Final Fantasy features the Crystal Tower as an endgame area. This iteration of the Tower is not based on the iteration from III, although the interior bares a resemblence.
- Final Fantasy XIV would homage the entire gauntlet (sans Eureka, which would get homaged much later with "Eureka Orthos") with the "Syrcus Tower" raid, albeit with the context shifted to fit within the world of Eorzea. The raid consists of "The Labyrinth of the Ancients", "Syrcus Tower" and "The World of Darkness" and feature many of the monsters/bosses from the original game. The Crystal Tower also plays a significant role in the Shadowbringers expansion.
- Stranger of Paradise: Final Fantasy Origin bases its 5th stage on the Crystal Tower, mixing it with the Mirage Tower from Final Fantasy (conceptually: In-universe, it's stated to be a prototype). Compared to other stages, there aren't too many specific references, but a particular area near the end seems to reference the mirror on the top floor. The game also subtly references III's theme of Wind, as the "Warriors of Light" must pass through the tower to reach the area where the Wind Crystal is kept.