One of the biggest additions in the big A Realm Awoken patch for Final Fantasy 14 was the Crystal Tower (CT), also known as the Labyrinth of the Ancients. It's a huge dungeon composed almost entirely of boss fights that awards 200 Philosophy Tomestones and 50 Mythology Tomestones, in addition to level 80 gear drops. You can only receive one gear drop per week (which resets every Monday morning in the U.S.), but you can pile up the tomes until you reach the cap.
At the moment, the Crystal Tower is one of the best ways to acquire tomes, but only if you can make it through in a timely manner. The dungeon requires 24 players separated into three alliances of 8 players each. Unfortunately, you can only go in with an alliance of up to 8 players. While some Free Company groups have successfully synced up their Duty Finders to get a full group of 24 players into the same CT run, it's not commonplace. Under most circumstances, you’ll rely on 16 strangers to help you through the Tower. Refer them to these tips so you make it out alive, and with some nice level 80 gear.
The first section leads up to the Bone Dragon boss fight. Stay on the elevated green platforms throughout most of this section to avoid poison that fills up the lower portion of the area through most of the fights. In the first portion of this run, players must battle against two Valefor dragons, almost as a warm-up. The key here is to have the tanks point the dragons inward. This avoids a party wipe from the AoE attacks both in front of and behind the dragons.
Once you reach the Bone Dragon, the common practice is to tank it on the northern-most platform, but it's also possible to tank it on the middle platform. The Bone Dragon must be killed three times, reviving itself after the first two deaths. The important part of this fight is not the boss, but the adds. Each tank should claim a row of green tiles as their own. Several Platinal skeleton adds appear while fighting the Bone Dragon. Each add needs to be picked up by a tank and taken to the far end of their previously claimed row of tiles. Once they reach the end, the DPS players need to kill the adds.
The Bone Dragon should not die before all of the adds have been killed, and adds should not be killed close together. That's why the tanks need to take them back to their designated areas before the DPS players taken them down. The adds respawn after the Bone Dragon dies and move toward the boss's previous location. If they reach the boss, it causes a huge AoE attack that inflicts significant damage to the alliance. If these two requirements are not met, the skeletons will gain a speed boost, making it very difficult to kill them before they reach the Bone Dragon.
There isn't much to the Atomos boss battle so long as everyone knows their role. There are three halls with a platform at the entrance to the hall, and another platform at the end where one of three Atomos bosses awaits. Four members of each alliance need to stand on the platform near the entrance. Atomos is invincible until at least four party members are standing on the platform. However, your platform does not control your Atomos. Instead, it controls the invincibility of one of the other Atomos bosses, so it's important to never leave the platform until it stops glowing, indicating the Atomos it controls has been killed.
In addition to fighting Atomos, several adds spawn from the platform Atomos hovers over. The two tanks should alternate between the adds, while two ranged DPS and the healers stand on the platform near the entrance to fight the adds. The remaining two DPS players should be attacking Atomos. If you have no ranged DPS, the tank should pull the adds all the way to the platform, but it's best to talk with the other alliances and trade players for this battle to ensure all parties have at least one or two ranged DPS classes.
The battle against Thanatos consists of three Magic Pots spread out around the outer edge of the room. Each alliance takes a Pot and is tasked with protecting it. The Pots allow one alliance at a time to gain the effect of Astral Realignment, which essentially means they're invisible. Thanatos can only be damaged by the invisible alliance. Similar to the Atomos fight, your Pot does not control the invisibility of your alliance members. For example, if you're in alliance A, your Pot may control the invisibility of alliance C.
The Pots can be treated like party members. Healers should cast Stoneskin on the Pots and cure them when necessary. The alliance that is invisible should attack Thanatos, while the other two alliances take care of the numerous adds that spawn and attempt to kill the Magic Pots. It's important to engage all of the adds, not just the adds around your Pot. Keep in mind, the alliance that's fighting Thanatos is not protecting their Pot. It's up to the other two alliances to keep their Pot alive until the invisibility wears off (60 seconds).
This is arguably the easiest fight in the Crystal Tower, but only if people take care of the adds as quickly as possible. There are three Vassago demons. Each alliance takes a demon as their own. Ignore the Allagan Bomb in the middle until all three Vassago demons are down. That means don't attack the Bomb, don't look at the Bomb, don't even think about fighting the Bomb.
During the fight against the demons, Allagan Balloons and Napalms spawn. The Balloons will aggro like any normal mob, but the Napalms make a beeline for the Bomb in the center. While it's important to take out all of the adds as quickly as possible, it's especially important when it comes to the Napalms. If the Napalms make it to the Bomb in the center, the entire raid is hit with a damaging AoE attack.
As long as the adds die quickly, the Vassago aren't much of an issue and should go down quickly as well. Once all three are down, the entire raid should focus on the Bomb to finish the boss battle.
This is a tricky battle, but as long as everyone does their job, this isn't difficult. There are four towers that line the outer edge of the area. One tank should be in charge of each tower. When it lights up, the tank should interact with the tower to essentially turn it off. This keeps the constant electricity on the ground from inflicting too much damage on the raid. Healers need to keep a close eye on the tanks and the raid as a whole because everyone will be inflicted with constant damage at all times. Medica II and Regen are exceptional spells to use during this fight.
In addition to King Behemoth, Puroborous bombs and Iron Giants spawn. The bombs spawn in the middle of the area and head for the towers. Once they reach a tower, they explode, damaging the tower. If a tower takes too much damage, it's destroyed and can no longer be turned off, which causes the entire raid to take more damage from the electrified surface you're fighting on.
It's important to take out the bombs as quickly as possible. Only one Iron Giant appears during each rotation, so have the off-tank move him to one side. The DPS classes should not attack the Iron Giant until the off-tank has him in place, and the off-tank should make sure the Iron Giant's frontal attacks are not hitting any towers.
During the battle, some players will have a green arrow over their heads. This indicates that a comet is about to land on them. Head to the outer edge of the center circle away from Behemoth and wait for the comet to drop. Once a comet has landed, stay away from it to avoid the fire damage caused when a comet explodes.
At the end of each rotation, Behemoth roars for several seconds and the text, "GRAAAARRRRGH" appears on-screen. This casts Ecliptic Meteor, which calls a giant meteor to engulf the entire area. You must stand behind a comet so that the comet is between you and Behemoth. This is your only protection against the giant meteor.
The best course of action is to have ranged DPS classes take down the bombs, and melee DPS classes take down the Iron Giant, then Behemoth once the Giant is down.
The final battle against Acheron is fairly easy. He uses quite a few AoE attacks, but as long as your healers are doing their job, there isn't much to worry about. Stay away from the brightly colored areas that market Acheron's AoE attack area, and you should be fine.
The only adds that spawn during this battle are Iron Claws and Iron Giants. When a full set of Iron Claws spawn, it's a warning that Acheron's raid-wiping AoE attack, Ancient Flare, is about to be cast. The Claws latch on to one party member, completely disabling him or her until it's defeated. When the Claws spawn, take them out quickly.
There are three large circles around the outer edge of the area. These are important when Ancient Flare is being cast because they cause a shield to form that protects the raid. At least four players must be standing on each of the circle platforms to form the shield.
Once the shield is up and Ancient Flare has been cast, an Iron Giant spawns on each circular platform. Make sure the tank is facing the Iron Giant away from the rest of the party to avoid its frontal AoE attack. After the first Ancient Flare, an Iron Claw spawns alongside the Iron Giant. In addition, a single Iron Claw will spawn during the battle against Acheron, but this does not signal Ancient Flare. Only the group of Iron Claws signals Ancient Flare is coming.
Take down the adds, watch out for Acheron's AoE attacks, and this fight is fairly easy!