IMO, whoever is commanding the wyverns (flying units) should've just been hitting supply lines and villages/farms so they can't supply the unoptimized fortresses. Unless they also got anti-air, fast units, or fliers of their own, they really can't do anything about this tactic. It also scatters the Demon King's forces trying to defend from multiple places at once, contributing more to supply consumption and tiring the armies out.