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.
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. The final fourth of the game revolves around the Warriors of 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 Labyrinth of the Ancients, 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 the 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 starts proper with the ten floor dungeon that has been built up since the middle of the game. There is surprisingly scarce information in the game itself regarding how the tower came to be, although the presence of the Ancients' Maze suggests that it predates the Flood of Light.
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.
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 save point 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 save 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.
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.