From Wikipedia "
The AT-AT is a large, four-legged walker introduced in The Empire Strikes Back and also appearing in Return of the Jedi.
Joe Johnston's original design for the Empire's war machines was a giant, multi-wheeled vehicle; this design later became the "Juggernaut" in West End Games' roleplaying material, and the design was reworked into the clone turbo tank for Star Wars Episode III: Revenge of the Sith.
For The Empire Strikes Back, however, the final design was a four-legged walker. Industrial Light & Magic (ILM) created models ranging from 6 to 50 centimeters in height. ILM filmed the AT-ATs using stop-motion animation against matte paintings created by Mike Pangrazio because attempts at compositing miniature footage against live-action background footage yielded mediocre results. Additionally, ILM studied elephants to determine the best way to animate the four-legged AT-ATs. Although the stop-motion animation style gave the AT-ATs a jerky, "staccato-like" effect on film, ILM found this movement acceptable because of the AT-ATs' mechanical nature. The sound of the AT-AT walking was created by Sound Designer Ben Burtt, by using the sound of an industrial sheet metal stamper.
Dialog in National Public Radio's adaptation of The Empire Strikes Back states that AT-ATs "look like animals"; the character goes on to describe the vehicles as carrying "extremely heavy armor and armaments". The AT-AT, designed to favor "fear over function", can carry five speeder bikes and 40 Imperial stormtroopers. The walkers themselves carry two blasters and two laser cannons. Manufactured by Kuat Drive Yards, Expanded Universe sources describe the AT-AT as being either 15 or 22.5 meters tall. Their armor is resistant to standard blaster weapons; however, the "neck" column of the walker holds no such invulnerability and, if shot, can cause the entire walker to be destroyed.