The war machines are enormous machines appearing as tall, steam powered metallic statues. They are larger than any wall that has been built, and they arent just powered by steam- they are powered by remnants.
Each of the six war machines that have been built require a remnant to work, remnants are hard to come by, as they are only taken off the corpses of individuals of godlike power, or at least close to that level.When in use the remnant becomes a heart for the war machine. Giving it life to move and slay.
It takes years to build a single war machine, and as stated before, the much required remnant to make the machine work is next to impossible to find without several brave souls and the knowledge of a creature who bears one. But once it is finished, it can destroy armies easily, even without the aid of an army of its own.
There are only six of the machines in working order, and not a single one has been destroyed, yet... Unfortunately Alexander's rebellion has managed to steal one of them, but not before one of the engineer clerics stole the remnant that powers it. But who knows if Alexander already has a replacement?