All Terrain Soup
All-Terrain-Walkers.
Legged metal bawkses. No, not those.
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.
The pseudoscience reason for walkers in Star Wars is that shields apparently "float" over grounded surfaces, so things can walk through shields, but ungrounded things can't float or fly through shields. Of course, if you can walk and stay grounded you can ROLL and stay grounded too...
But that's not why the empire uses them. They use them because they're intimidating as fuck giant walking metal bawkses.
AT-RT
Motorcycles with legs AT-TRs are fast moving recon units. A very prolific unit used both by the GAR and the Rebel Alliance, as well a a few outlaw bands. There hasn't been a definitive reason the Empire phased them out, maybe they got out bid?
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 its 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; hardpoints on the sides can mount various heavy infantry weapons. Vulnerable to attacks from tribal teddy bears and conveniently-placed log traps.
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 its points back, but did an excellent job tanking for the rest of your army.
In Legion
Content.
AT-AP
The Ares to the AT-TEs Land_Raider. The Attack Pod replaces the dorsal mass driver with a heavy laser, slings a cannon underneath to cover the belly now that the whole hull has been put of two stilt legs, and then replaces the troop bay and forward cannons with Heavy Mass Driver. A third leg is retractable to add stability.
AT-AP on the Table top
No game has ever tried to put these things in, even most videogames shy away form them. However with the TX-130 already released the GAR is running out of "mid-sized" vehicles to make models of so we might see it. At more than twice the mass of the AT-ST it would be the biggest model in the game and would compare favorably to the likes of the Baneblade for sheer size.
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 camels, 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 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 enormous legs are one of the vehicles defining features, and seem like weak points the truth is a bit more complicated. Ion (EMP) rockets and missiles large enough to threaten AT-ATs are rare and mostly built by Imperial manufacturers, mines large enough to do the same are not. The necessary sections of the legs are all mechanical, there are sensors and other things that are nice to have, but if a leg takes an EMP the walkers motive systems are unaffected.
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 disembark they must use jump packs or drop ropes. 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).
Though it is worth remembering in Star Wars setups like Echo base are not uncommon. Unrestricted by size and power, ground based cannons and shields are vastly superior to their space mobile counterparts. After the Naboo attacks a lot of planets started self fortifying, and even more got scary stuff from either the GAR or CIS. The AT-AT is a super heavy IFV for cracking these sorts of defenses. Poor Commanders (and writers) use them as MBTs but they aren't. The former exists because the Empire focused more on loyalty then skill in training and the latter because money is more important than accuracy( and because some legitimately good writers just are not well versed in military strategy.)
During the battle of Hoth Rogue Squadron's predecessor Rogue Group, 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 multi-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, had an even bigger cannon on its back.
Gallery
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Size Comparison
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After the Empire fell apart a ton of their stuff was left just lying around...
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AT-TE going in for a combat drop.
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More stolen goods
First Order Variant Gallery
The First Order had a bunch of Fanboy variants of the walker's built.