All Terrain Armored Transport

"They're the most heavily armored ground vehicles in the Imperial Army.''" "''...so what you're saying is that you have a thorough knowledge of just how screwed we are."

- Thane Kyrell and Yendor, just before the Battle of Hoth

The All Terrain Armored Transport (AT-AT) was a four-legged combat walker used by the ground forces of the Galactic Empire. With the Imperial-class Star Destroyer and the TIE/LN starfighter, it was one of the most famous symbols of the Empire's military might.

Description
"Watch out kid, this thing handles like a drunken bantha."

- Han Solo, to Luke Skywalker while piloting an AT-AT

The All Terrain Armored Transport was an imposing, quadrupedal combat walker that stood 22.5 meters tall. Due to their impressive appearance, it was used as much for psychological effect as for tactical reasons. Additionally, it also served as transports for combat personnel. The vehicle's cockpit and sensor array were located in the "head" part of the walker, which also carried the main armaments: dual medium blasters located on the "temples," and a pair of heavier laser cannons fitted on the "chin." Inside the cockpit stood an AT-AT commander behind two AT-AT pilots who controlled the walker's movement. Due to the AT-ATs lack of climate control, AT-AT pilots and personnel wore specialized suits in case the walker's pressurized cockpit was smashed open in hostile environments. The armor plating of the AT-AT was impervious to blaster bolts, however the neck and joints proved weak spots and could be damaged by blaster cannons and other heavy weaponry. The entire walker was vulnerable to other AT-AT fire, if one happened to be hijacked and turned against the other.

Connecting the "head" to the "body" was what was dubbed the "tunnel," which would lead to a troop compartment capable of ferrying numerous soldiers, along with a vehicle bay at the walker's rear that held five 74-Z speeder bikes. The main, rectangular troop hatch was located in the middle of the walker's side, while two smaller escape hatches were located both to the main hatch's immediate left and right. The walker's tail end consisted of filtered atmosphere intakes on its upper part, while a blaster cannon energizer was located within. At the bottom of the behemoth was the walker's fuel slug tank, which had a reputation of being dangerous. The walker's four legs were aided with the assistance of ankle drive motors, while inside the giant footpads were several terrain scanners.

There was a more heavily-armored variant with a darker color scheme called an Elite AT-AT.

History
"I heard of these. Imperial armored transports. Giant four-legged things that were used to keep the peace on conquered worlds."

- Scoggan, to Davjan Igo upon entering the ruins of an AT-AT

The AT-AT walker was developed for use in the Imperial Army, the ground forces of the Galactic Empire. It was the successor of the All Terrain Tactical Enforcer that saw use during the Clone Wars, although it constituted far more imposing example of walker technology. Heavily armored and awe-inducing, the AT-AT positioned itself as the standard of Imperial firepower on the battlefield.

In the fourth year before the Battle of Yavin, the Imperial Security Bureau Agent Kallus led three developmental AT-AT walkers on an assault on the desert planet of Seelos. These early AT-AT's were in fact larger than what would later be used by the Galactic Empire, and carried at least 1 614-AvA speeder bike in its vehicle bay. The 614-AvA speeder bikes would later be replaced with the newer 74-Z speeder bikes coming into production.

AT-ATs were used again sometime later against the Lothal rebels, when Princess Leia Organa arrived on the planet to give the Ghost's crew three Hammerhead corvettes. Two walkers were deployed to patrol the transports' landing platform, which were formally on Lothal to offer supplies to citizens impacted by the rebel efforts. When the rebels tried to steal the corvettes, the Ghost started firing on the AT-ATs. Four proton torpedos were not powerful enough to destroy one of the walkers, and it was necessary for Kanan Jarrus to use his lightsaber to cut its legs, toppling it. The second one was knocked down by the engines of the last Hammehead, piloted by Ryder Azadi.

Following the Battle of Yavin, during the Rebel assault on the Imperial weapons factory moon of Cymoon 1, a group of Rebels led by Han Solo and Leia Organa stole an AT-AT from Weapons Factory Alpha. They used the walker to escape from the factory after they sabotaged its main power core to explode. During their escape, Solo fired the shot that nearly killed the Dark Lord of the Sith, Darth Vader, using its powerful quad laser cannons. Soon after, Vader managed to overtake the Rebels in their escape and cut down the massive walker's back-left leg. The AT-AT fell tremendously and its passengers barely managed to escape the wreckage; before fleeing Cymoon 1 entirely for the safety of the Alliance Fleet

During the Battle of Hoth, General Maximillian Veers led a squadron of at least five AT-ATs on the icy fields of Hoth to knock out the shield generator that protected Echo Base, the Rebel headquarters. The triumph of Veers' squadron allowed the Imperials to invade Echo Base and rout the Rebels, striking a major blow in the war. However, the battle also revealed that the AT-ATs could be toppled by a cable wrapped around their legs. Although that weakness had been used effectively by the Alliance on Hoth, the Empire continued to use these mechanical behemoths in the later stages of the war, employing them in several skirmishes on Tatooine and later on Jakku.

Behind the scenes
The names "All Terrain Armored Transport" or "AT-AT" is never spoken in any of the films. Instead, the machines are referred to as "Imperial walkers." The AT-ATs used in The Empire Strikes Back and the brief appearance in Return of the Jedi were created with the use of stop-motion animation.

In the DVD commentary to The Empire Strikes Back, George Lucas explains that he got the idea of the AT-AT from the massive tripods from H.G. Wells' 1898 novel The War of the Worlds.