Although tactically questionable, the lumbering AT-AT walkers employed by the empire against the rebels on Hoth made for great action set pieces and an unexpected opening to a film which had principally taken place on a desert planet and in space.
The concept department on The Empire Strikes Back went through a number of concepts before they landed on the final form; a form which borrows heavily from at least two sources in particular. Here is the initial concept, which I think it's safe to say would probably not have aged quite as gracefully as the final AT-AT design has.
The walkers were indeed inspired by The War of the Worlds, more so than Lucas let on. Here's the cover for the 1962 edition, by Virgil Burnett seen next to Joe Johnston's no 108.
That's only half the story of course. Here's an excerpt from Syd Mead's book Sentinel (1979):
The four-legged, gyro-balanced, walking cargo vehicle shown on pages 80-81 is from the US Steel Interface portfolio series [published in 1969 - see image below]. The environment is arctic and the mission is to deliver goods and critical supplies to an isolated exploratory colony beyond the DEWline. Like several of the preceding designs, the 'feet' can be rotated and locked to form powered wheels for rolling over smooth terrain but, seen here in the walking mode, they are covered with ice that is breaking up into radial slivers as the pneumatic pods flex. Defining this part of the design concept more closely, Mead explains that "the largest land animal now extant is the elephant. As he puts his weight on each foot the metacarpals and tarsals fan out from the ankle and the foot spreads, distributing the pressure. Conversely, as the weight is retracted the foot contracts and never gets stuck in the mud. The same principle was incorporated here; the 'foot' structure would be alternately inflated and deflated in the walking mode to duplicate the natural function of the elephant's foot. As a matter of fact, when I did this the US Government had already funded a military project for a walking machine [RH-2010 - the GE Walking Truck] and had built an analog computer-co-ordinated prototype that successfully walked over loosely stacked railroad ties. The seated driver not only operated extremely sensitive hand and foot controls that duplicated and amplified his motions, but also had calculated feedback that allowed him to 'feel' the feet making ground contact!"
This particular Syd Mead painting appeared in the 1969 US Steel - Portfolio of Possibilities (Syd Mead made something of a habit out of mechanical walkers over the years, including moon walkers from the 1984 book Oblagon: Concepts of Syd Mead), a portfolio Joe Johnston got a hold of during the production of The Empire Strikes Back.
"George Lucas and Gary Kurtz knew at the outset that there was going to be a snow battle, and they knew we were going to have armored speeders. But they hadn't really decided on what kind of vehicles the Empire would have or how they were going to do it. At first they considered using existing military tanks from the Norwegian army, redressing them to make them look alien. I did a bunch of sketches using the tanks as a basis. Then I ran across a xerox that a friend of mine had. It was a promotional brochure put out by U.S. Steel in the early Sixties and contained a whole slew of full-color paintings indicating 'what still will be used for in the future.' The paintings were done by Syd Meade (sic) (Star Trek conceptual artist). Interestingly enough, one of the paintings showed a four-legged walking truck! That's here the initial walker idea came from. It was a very unique design."
Meade's (sic) rendering reflected the rich imagery of the science fiction pulps and could have easily been a surreal version of a Popular Mechanics cover. Johnston took that design and started militarizing it, giving it guns and a separate head. He also used copies of some design sketches by Ralph McQuarrie as an added influence. During the selection process, Lucas, Kurtz, McQuarrie and Johnston gathered in a room. Lucas would extract a few sketches from the batch and comment on the design features he desired. Eventually the final design of the snow walker was realized. "It was more or less up to me." said Johnston. "It amounted to a few sketches. Ralph at that time, was going to England to do some paintings and full-size sets, so he wasn't too involved in the end of the design phase. The walker you see on the screen is basically my design, with some borrowed elements from the original painting."
— Cinefex #3, p6.