All Terrain Soup
All-Terrain-Walkers.
Star Wars's Imperial Military and the Grand Army it usurped really liked their acronyms (much like their real world counterparts). The most egeriogus examples being the assorted combat walkers, which all start with AT, an acronym for "All Terrain", which is supposedly one of the justifications for the legs in the first place. Note that the AAT is not a part of the soup, being "Armored Assault Tank". Star Wars being Star Wars we know that most, if not all, of these walkers are produced by Kuat Drive Yards and there subsidiaries Rothana Heavy Engineering,
AT-RT
AT-RTs On the table top
AT-PT
The oft forgotten (since it debuted in Dark Force Rising instead of the movies and didn't have a clear description till the Dark Force Rising Sourcebook) little cousin to the AT-ST. For the most part the role it once had in the story has been replaced with the AT-RT, since that fits the role of "small pre-Imperial walker" but actually appeared in the movies. The modern (3E onward) Sentinel looks like one with less armor.
AT-DP
All-Terrain-Defense-Pod. An early design for the AT-ST that was reused by Disney. No clear reason for it to exist was ever given besides Disney wanting to have a different looking AT-ST. Possibly to have a light walker that was more intimidating than the AT-RT, but not have letting AT-ST lose it's rep to being slapped around without too much effort.
In Rebels (it's only appearance so far) it's less than ideal, unable to gain even a temporary victory because they're the bad guys in a Disney cartoon, who aren't Thrawn.
AT-ST
"All Terrain-Scout Transport", aka "The Chicken Walker", aka "the thing that the Imperial Guard Sentinel is a shameless rip-off for". Basically a box-shaped control room built around a central heavy laser gun and mounted on two spindly legs.
AT-ST on the Table-Top
In RPGS
AT-STs are a quintessential part of a Star Wars campaign. They are an excellent choice for and GM who would like to avoid the big bad getting blasted apart by your PCs hotshot Corellian gunslinger or the Mandalorian you somehow allowed in the game, without just having all the boss fights be in space. Big and scary without being completely out of the scale a party can reasonably deal with, though if you party lacks a Jedi or heavy weapons they might need to get creative, which the source material provides many examples of.
D20 Star Wars Minis
In addition to looking Derp as all get out the AT-ST was something of a middling unit, It was very tough and had DR10 but despite hitting hard it struggled to actually land those hits combined with how confined maps in usually were you had a unit that could struggle to kill it's points back, but did an excellent job tanking for the rest of your army.
In Legion
Content.
AT-AP
Artillery.
Essentially a larger AT-ST built around a cannon with with a third leg for a crotch that can deploy to stabilize the whole thing into a tripod configuration for firing. Seen for like a second or two in Episode 3.
AT-TE
Spider Tank.
Low slung and slower moving, the AT-TE is a pure infantry support walker with four blasters forward, two aft, and a dorsal fuckoff gun. Designed to be airmobile, lifted and dropped by a skycrane variant of the LAAT gunship.
AT-AT
"All Terrain-Armored Transport". These are the big, BIG bastards amongst the All-Terrain Walkers; enormous quadrupedal armored transports who look almost like giant robot cows, but who aren't so funny when you realize they can shrug off heavy artillery like it was nothing. Slow as molasses, but they will get to where they're going, and the head-like cockpit's twin heavy lasers can dish out a fair bit of pain along the way. Slow is something of a misnomer whilst the the movements are far from swift long legs can cover a lot of ground, allowing the walker to make nearly 40mph (or 66kph). Not quite speedy but more than enough to cover a lot of ground to deposit it's complement of troops without taking too much fire.
The AT-AT doesn't have a good parallel in modern armored fighting vehicles. It has fairly powerful weapons, but only in a narrow arc. It has capacity for a couple squads of passengers, but to embark or disembark they would need jump packs or ziplines. Its principle virtue is simply its absurdly heavy armor, such that it is effectively impervious to the scale of energy weapons that it could reasonably expect to face on the battlefield. In the face of massed blaster fire, the AT-AT simply plods along, smashing through resistance by virtue of being little more beyond a gigantic walking slab of metal. It's essentially a battering ram; a combination of assault gun and heavily armored APC for when you have to face a defensive line that would impress the French in order to bring down a high priority obstacle (like a shield generator).
During the battle of Hoth Rogue Squadron equipped with modified T-47 Airspeeders were able to destroy several walkers. The T-47 is based on a ship yard tug, refitted for cold weather combat. They kept the harpoon launcher from the default model. The harpoons were capable of adhering to the armor of walker using specially designed magnets, the cable had the tensile strength to haul around mult-ton ships mid construction, so with several loops were able to trip the walkers. However they suffered heavy losses during the battle and were unable to stop Blizzard Squadron form destroying the shield generator and overrunning Echo Base.
AT-AT On IS the tabletop
AT-ACT
The armored Space Truck of Empire, possibly made from the repurposed hulls of the old model AT-AT seen in Rebels. They haul important things from one place to the next.
AT-M6
The First Order's Bigger Scarier Gorilla walker.
Gallery
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Size Comparison -
After the Empire fell apart a ton of their stuff was left just lying around... -
AT-TE going in for a combat drop.
First Order Variant Gallery
The First Order had a bunch of Fanboy variants of the walker's built.