They are unlocked at HQ Level 15, and is the last unit class to be unlocked in the game.
Combat Guide Edit
The walker is a formidable unit class - even the Tanks can't fight the Walkers properly if they are not commanded properly. However, this unit class still has one very deadly Achilles' Heel, and, upon active player control, Tanks may even have a chance to beat Walkers.
The very first impression when new players face the Walkers is their extreme range. But, upon closer inspection, the Walkers also possesses a very large blind spot. In fact, the blind spot of the Walkers is the greatest of all the ground units. So, when someone manages to get under the Walkers' firing range... Congratulations. Your army of Walkers are pretty much badly damaged.
As you can see, Walkers cannot fire too close, so sending some units under the blind spot can be a good idea. Opponents usually attempt to take down Walkers with, in terms of number of armies, not number of units, overwhelming forces so that the Walkers cannot protect themselves from multiple attacking forces properly.
As you may also notice, the Walkers are also very expensive to build, and think about that: you may want to rebuild your Walker army someday with fastest speed possible. Therefore, defenders with Walkers usually "conserve" the Walkers and position them behind the cannon fodders. They may try to have the Tanks cover the blind spot, or place multiple Walker armies together so flanking attempts can be discouraged.