Celestial Pillars

The Celestial Towers (aka Pillars) are four bosses that appear during the Lunar Events, at 4 predetermined spots on the map (2 at the west and 2 at the east). The Pillars only appear after the Lunatic Cultist has been defeated. The 4 Celestial Towers are the Vortex Pillar, the Stardust Pillar, the Nebula Pillar and the Solar Pillar. The craftable gear from each Pillar correspond to the 4 main classes in the game; Vortex for ranged, Stardust for summoner, Nebula for magic, and Solar for melee. The Celestial Towers all drop between 12 - 90 fragments each, associated with their theme, depending on your world mode.

At the beginning of the battle, the Celestial Towers have a large force field around them, which protects them from any sort of damage. In order to lower the field, you need to defeat 100 minions (150 in Expert Mode) that correspond to that particular Pillar. After doing that, the Pillar can be damaged. Minions will continue to spawn until the Pillar is defeated. If you exit the game without destroying a Pillar, all damage done to it will be reset, and the minion death count will also reset.

The bosses themselves are fairly easy, having only 20000 HP each and dealing no melee damage on contact. While they may not be a direct threat, the minions they spawn can deal high amounts of damage, move very fast or shoot fast moving projectiles, inflict debilitating debuffs, and can easily swarm the player if not dealt with swiftly.

Defeating each Pillar triggers the following status messages:
 * First Tower destroyed: "Your mind goes numb..."
 * Second Tower destroyed: "You are overwhelmed with pain..."
 * Third Tower destroyed: "Otherworldly voices linger around you..."
 * Fourth Tower destroyed: "Impending doom approaches..."

After defeating all four Celestial Towers, the screen will start to vibrate and slightly dim and the music volume will lower, indicating that the Moon Lord is about to spawn. It will spawn after about a minute.

Vortex Pillar
The Vortex Pillar is the ranger-themed Celestial Tower, with most of its enemies being able to fire projectiles and move quickly. When nearby, the background will display a green moon with lightning. It drops Vortex Fragments after it's defeated.

It will occasionally summon a similar projectile as the Lunatic Cultist where he summons an immobile, giant Magnet Sphere that shoots lighting at the player.

The minions spawned by the Vortex Pillar usually move fast, fly towards players, and attack with long ranged projectiles. The Storm Diver and Vortexian are usually easy to deal with, however, the Alien Queens can fire stingers that cause the Distorted debuff, which causes the player to be unable to fly upwards or fall downwards for a few seconds, by distorting gravity around them. This forces the player to only be able to move horizontally, and make them easy targets for attacks from all of the minions. The Alien Queen can spawn Alien Larvae, which will grow to Alien Hornets and into more Alien Queens if you leave them for too long.

 Summons :

Stardust Pillar
The Stardust Pillar is the summoner-themed Celestial Tower, with most of its enemies spawning smaller entities to attack. When nearby, the background will display a cyan moon and a sparkling effect. It drops Stardust Fragments after it's defeated.

The Stardust Pillar is able to spawn enemies itself via a special animation. The Stardust Pillar will spawn multiple spheres of energy, and will soon link together with a thin line and the line will quickly disappear. Shortly after, the energy spheres will disappear and spawn in an enemy.

Some of the minions spawned by the Stardust Pillar have abilities or attacks that occur after they die. The Star Cell enemy, if killed, will split into smaller sized versions that grow into full sized versions after a few seconds, which can cause players to unwittingly fill their screen with enemies. This can be very dangerous if the player can't dodge them effectively, or attack multiple enemies at once. However, leaving the baby Star Cells alone is an effective way to remove the Pillar's shield more quickly, since the full-grown Star Cells will contribute to the lowering of the shield when killed. The Flow Invader will summon smaller creatures around it, which can shoot a volley of arrows like projectiles at the player. The smaller creatures will shoot themselves towards the player after you kill the Flow Invader.

Summons :

Nebula Pillar
The Nebula Pillar is the magic-themed Tower, with minions that are able to fly, teleport, and fire complex patterns of projectiles. When nearby, the background will display a purple moon with nebula-like effects, such as beams of energy pulling debris into the sky. It drops Nebula Fragments after it's defeated.

It's the only Pillar that doesn't have any kind of attack, although you should still watch out for its minions, especially the Nebula Floater and Evolution Beasts, as their projectiles can move very fast and home-in on the player, respectively. They also deal large amounts of damage. It may be best to run from all of the minions, as the Brain Sucklers can attach themselves to you, blocking your vision and dealing damage, and the Nebula Floater will teleport towards you, making it more difficult to dodge its projectiles. While near this boss, the player's screen will take on a dull purple shade, tinting all blocks, players, monsters, and the sky purple.

Summons :

Solar Pillar
The Solar Pillar is the melee-themed Tower. Many of the enemies that spawn near it are melee-themed, with strong defensive abilities, heavy contact damage, and few projectile attacks. When nearby, the background will display an orange moon, with occasional falling fireballs. It drops Solar Fragments after it's defeated.

The Solar Pillar will occasionally spawn fireballs from it's "mouth", which deal heavy contact damage.

The Crawltipedes that the Solar Pillar spawns can be devastating to players using flight, as they deal high damage and are very agile. However, despite their high health, the damage they take is multiplied by 10, and they stop attacking players that land on the ground. The damage multiplication makes healing weapons like Vampire Knives extremely effective. They can only be damaged on their tail, where they have an orb that gets redder when they take more damage. The Srollers can deal abnormally large amounts of damage if they land directly on the player, the Corites deal larger amounts of damage and move faster if the player stays in the air for long, plus they can travel through blocks, making them harder to escape without coming across Crawltipedes, and the Selenian can reflect certain projectiles back towards the player and roll towards them at high speed, which all can make the fight very dangerous out in the open.

Summons :

Achievement
Star Destroyer is the achievement you earn for defeating all four Celestial Towers for the first time.

All Pillars

 * A quick and effective way to kill all of the Celestial Towers is to use the North Pole directly and vertically above a Pillar. Even if a player does not have any melee modifiers, if all of the snowflakes can make contact, the Pillar should fall in less than 20 seconds.
 * Another strategy is to use the Nimbus Rod as you can summon two above the Pillar and let it deal constant damage. If you leave a nimbus cloud above the tower and teleport away, it will still do damage until either the tower dies or the cloud runs out. This tactic can be combined with Spectre Armor and a Spectre Hood for long distance healing, as the healing effect has a very long range.
 * You can also use Minions (or a Spore Sac) while taking cover in a hole to do a safe kill. Take a note that some enemies can travel through blocks.
 * Some other weapons might be more effective, but they are not obtainable until the first Pillar is destroyed. Therefore, if you decide to start with the Vortex Pillar, using the Phantasm will be incredibly effective for the other Celestial Towers. The Solar Eruption (from the Solar Pillar) also deals massive piercing damage and is very effective as well.
 * Some Pillars and their enemies seem to be weak to their own weapons. For example, the Nebula Pillar is weak to the Nebula Blaze and Nebula Arcanum.
 * The longer the time taken to defeat a Pillar, it will sink deeper into the ground, but very slowly.
 * Repetitively throwing Vampire Knives will keep you healed most of the time, and if you get to less than 150 hp, use a Greater Healing Potion or higher. This will slowly take out the enemies and the pillar itself. Switching to another weapon is a bad idea, unless you have full Spectre Armor with a hood, though following this strategy, you should have Beetle Armor.
 * Using the maxed Attack Nebula buff with the Lunar Flare, around 3000 DPS can be dealt on a pillar, effectively killing one in seconds.
 * The shield surrounding the Celestial Towers becomes more transparent as it becomes closer to breaking. This is a good indicator of when the shield will break and leave you able to damage the Pillar.
 * The hardest Celestial Tower to deal with is the Nebula pillar as it is the only pillar whose enemies can deal massive damage, even if the player is underground or in a bunker. * The easiest way for players to deal with the Celestial Towers is to use a weapon which can hit through blocks, such as the Death Sickle, and mine tunnels starting from just outside the event zone until you can reach the pillar with your chosen weapon. All that's left to do is to grind though any enemies which converge above your location.
 * It is strongly recommended that you take down the Solar Pillar first as it will allow you to craft the Solar Eruption which will speed up the process.
 * Alternatively, defeating the Nebula Pillar first will allow the player to craft a Nebula Arcanum, which with a good modifier can take down all of a pillar's HP in the amount of uses it takes to use up all the player's mana. Using certain armor sets and/or equipment with the arcane modifier will make mana last longer, more effectively defeating the Pillar and remaining enemies.
 * You can use the GPS to know the depth of one Pillar then build/expand your nearest building of the pillar to this depth and shoot on it with Rocket Launcher with Rocket IV then Rocket III or Sniper Rifle. Rocket IV will destroy the obstacles and Sniper Rifle/Rocket III will kill the Pillar. The time who take the rocket to hurt the pillar can however be long : in large map, a rocket IV takes ~1 min to travel from the center of the map to the farthest Pillar.

Vortex Pillar

 * If you see a swirling portal form in midair, keep your distance and move quickly. The portal will either shoot a lightning bolt similar to the Lunatic Cultist's attack or spawn an Alien Hornet. When the portal shoots a lightning bolt, there will be a blue swirling marker pointing towards the direction the lightning bolt will strike at. Lighting bolts will only shoot downwards. The Vortex Pillar will always spawn Alien Hornets on back-walls with a direct line towards the player if there are any on the player's screen. of not, it can spawn Alien Hornets anywhere within an approximate 300 block radius of the tower.
 * Because most of the enemies try to stay at the same elevation as the player, with the exception of Alien Queens, staying on the ground removes much of the mobility that Hornets and Storm Divers have.

Stardust Pillar

 * Using splash-damage weapons on Star Cells is perhaps the easiest way to kill them, as it will destroy all of the split cells as well as the initial cell.
 * A quick and easy way to break the shields (especially on expert worlds) is to lead a star cell away from the tower, pop it into its smaller orbs, then let those orbs grow into full star cells. This creates an infinite loop, and the new star cells will also count towards the breaking of the shields.
 * Arguably, the Stardust Pillar is the easiest of the lot; its enemies are predictable and slow in their motions, and with focus, even the most menacing of the lot, the Flow Invader, can be easily countered.
 * On death, the Flow Invader will hurl up to three projectiles directly at you (potentially fewer if it is freshly spawned and hasn't yet accumulated its full contingent of orbiters). Be ready to dodge when you kill this creature.
 * Milkyway Weavers can be easily slain by using splash-damage weapons on them as soon as they burrow out of the ground. This will ensure that even if your projectile doesn't strike their head directly, it will still deal splash damage as it hits the ground next to it.

Nebula Pillar

 * When fighting, take caution to avoid getting hit by a Brain Suckler. They cause the Obstructed debuff, which is similar to Blindness but with a darker shader. It will persist until you kill the Brain Suckler stuck to you, and for a moment thereafter.
 * Be very wary of Nebula Floaters. They do high damage with high accuracy, and their tendency to teleport - especially when harmed - can make them unpredictable. Nothing prevents a Floater from teleporting directly in front of you and firing at point blank; if you wouldn't survive another shot and can't heal up, it may be best to retreat and recover.
 * Kill Evolution Beasts quickly, because once they get in range they will fire a slow but highly damaging ball of energy that tracks the player, passes through walls can deal continuous damage, and only expires after a few seconds. These will force the player to move around into other hazards and enemies.
 * Keep on eye for Predictor because they will stay from a distance, not attempting to get close to the player, and shoot the Nebula piercer.

Solar Pillar

 * While fighting the Solar Pillar enemies, it is highly recommended to stay grounded. Airborne players are easy prey for the Crawltipede minions, who do an excessive amount of damage but only attack airborne players.
 * If you have a skybridge set up in your world, standing on it will allow you to shoot down at enemies on the ground and even the pillar without being targeted by the Crawltipedes. Keep in mind Srollers will still be able to jump up to the platform and roll towards you.
 * If you have already problems killing the enemies to get the shield down, an easy solution is to dig a tunnel under the Pillar and use weapons that go through walls like the Death Sickle or the Nettle Burst. It will help a lot using the Spore Sac (Plantera Expert Item) and a random minion. You can also kill the Pillar using a wall-piercing weapon. If it is stuck on a hill, you can build a non-platform bridge to it without being assaulted by Crawltipedes. It may be wise to prioritize Corites, as they can and will attack you without regard to intervening terrain.
 * If you are low on health, and you are wearing wings, a good strategy for regaining health is to alert a crawltipede and hit it with Vampire Knives. Due to the fact that they take ten times as much damage as any other enemy and have a high amount of health, it is a good way to build up health fast; provided you can dodge the Crawltipede itself. However, if you are struck by the Crawltipede with low health, you will most likely be slain.
 * It is highly suggested not to use bullets, arrows or Meowmere, at least not on Selenians as they can reflect bullets that deal the same damage they would have dealt to the enemy. Powerful weapons of those varieties can easily be turned against you and you can die very quickly. Selenians, however, cannot reflect Rockets, so it is suggested to use those against them if you are to play ranged.
 * If you have a Neptune's Shell, an combined accessory with its effect, or Gills potions, then standing in a large pool of water under or near the pillar will allow you to move around off the ground, without being targeted by Crawltipedes. In addition, this will also cause most enemies, with the exception of Corites, to move slower, and make it easier to target them. Srollers seem to stand still in the water, making it incredibly easy to target them. This strategy also works well with Vampire Knives, allowing you to heal more efficiently. Can be done easily with pre-Plantera armors, such as Chlorophyte.
 * If available, the Scutlix Mount improves forward firepower and ground maneuverability considerably, making the fight much easier.
 * When fighting the Solar Pillar, try building a small building with doors, platforms, and make it two floors tall, right on the edge of where the tower starts spawning enemies, to help block off some enemies from swarming you(especially the Srollers who can deal very large amounts of damage on Expert Mode), and to force them into a small and easy to defend area. Only the Corites will fly through walls.

Pillar Spawning Mechanics
When it is time for the 4 pillars to spawn, the game follows these steps: (The coordinates are in tiles)

(For example, a large world is 8400 tiles in width, so the 4 pillars will be spawned at somewhere near x=1680, x=3360, x=5040 and x=6720)
 * 1) Randomly arrange the order of the 4 pillars.
 * 2) Multiply the width of the world by 0.2, 0.4, 0.6, and 0.8 to find rough spawn locations for the pillars, equally spaced from each other and the world's edges.
 * 1) For each pillar:
 * 2) Pick a random x1 coordinate in the range [x0 - 100, x0 + 100], where x0 represents the x coordinate of the location the Pillar was assigned to.
 * 3) Try to find out a possible y1 to spawn the pillar at (x1, y1). The y1 is processed from the 0' surface point to somewhere 100 tiles below the top of the world, from low to high. This place is a valid spawning space for a pillar if:
 * 4) * There are no solid blocks (doesn't include platforms and other similar tiles) in the rectangle of x coordinate [x1 - 10, x1 + 10], y coordinate [y1 - 20, y1 + 15] (the smaller the y is, the higher height the spot lies)
 * 5) * The pillar will be outside the player's sight, checking whether tiles (x1 - 10, y1), (x1 + 10, y1), (x1 - 10, y1 - 20), (x1 + 10, y1 - 20) are all outside a rectangle centered at the player with a width of 1920*1.2 pixels and a height of 1080*1.2 pixels.
 * 6) If a y1 works then the pillar spawns at (x1, y1).
 * 7) If none of the valid y1 works, go back to the first step
 * 8) If all steps have been run 15 times and failed, give up and just spawn at a settled point: (x0, y0 - 40), in which case the Pillar will always spawn at 80' Underground at a fixed x coordinate.

Trivia

 * The Nebula Pillar is the only pillar to not have an attack that damages directly, as the Solar Pillar attacks with solar fragments, the Vortex Pillar summons portals that spawn Alien Hornets, and the Stardust Pillar attacks by summoning white portals that either spawn Star Cell, Milkyway Weaver or Flow Invaders.
 * Each pillar applies a shader to the screen, which makes it easy to distinguish when the player has entered a Lunar Event zone.
 * The Celestial Towers, Lepus and Turkor the Ungrateful are the only bosses that don't drop trophies.
 * All of the towers have a humanoid enemy. Vortex Pillar: Vortexian and Storm Diver, Stardust Pillar: Stargazer, Solar Pillar: Selenian and Drakanian, and Nebula Pillar: Predictor.
 * If the Lighting mode is set to color or white, the shield will have a distort pulse effect, making players around it have a shockwave effect. And when the shield is down, the area nearby get the shockwave effect. When the lighting is on retro or trippy, these effects won't occur.
 * When in a lunar area, depending on the pillar, the entire area nearby will turn a color. (solar-orange, vortex-teal, nebula-magenta, stardust-blue) (the effect will only occur if the lighting mode is on color or white.
 * When switching lighting modes, the stardust pillar is the only pillar with a background with the color that's not another shade. (the moon on color and white is blue, on retro and trippy it's white, the color shade in the background is light blue on color and white, and purple on retro and trippy.
 * Vortex Pillar is the only pillar that have planet (moon) with ring, and may have a references to Jupiter.

History
Tour Céleste