This is overthinking it. Willow is neither medieval or post-apocalypse. It's quite deliberately ahistorical, which is why they put a piece of modern music in each episode to drive that point home.I think the person who said they thought that the world of "Willow" was post apocalyptic might be on to something. Did anyone take a good look at the ruined city where Airk is being held? Would would a fantasy "medieval" city need 6 lane roads, let alone overpasses? It also has a modern style grid layout and a distinctly high-rise skyline.
Weird alien city is weirdly alien. GenericFantasyland is full of them.