One of the White King’s great assets – even though he possesses a fallen Angel in his ranks was his belief that faith alone would not save his army and the art of both mechanoid and wizardry could always be relied upon to be great allies when administered carefully.
Hence the reason the king kept Universities that studied the mechanical and more unorthodox majical arts.
It was through these academies principled by the Queens Bishop that the flank engines were created.
The Steam City
The Left flank that provides the Queen’s armies protection is a massive structure made of slick marble and iron making the structure unscaleable except from within. The ability to create what can only be described as a turret on wheels also provides a means to carry the food, supplies and weapons that a large army needs to conduct a battle of mythical proportions.