I'm going to jump straight into a very small description of the world my first novel-thing is build around.
It's an Earth-like planet orbiting a binary star. Climate is slightly more extreme than on Earth (think colder winters, hotter summers) and the ~800 million inhabitants adapted quite well to it. Technology is on a late-medieval level, compare it to ~1400-1425. Magic IS a thing on this planet. Everybody can learn to use it to the same extent, though it requires a lot of studying and memorizing runes. The strongest incantation documented was cast by a famous scholar almost a thousand years ago and since then, people came close, but could never reach that level. What was the incantation? Well.. he summoned an orb of fire the size of what we on Earth know as a Hippopotamus. That's it. Magic can levitate small objects, too, but its nothing too frightening or too dangerous at any time.
My "Villain", the Protagonist, manufactured with some aspects of Peter's Evil Overlord List in mind, can summon a fireball the size of a Chihuaha. It takes about 8 hours to prepare and exhausts him. So he does not use it. At all.
He'd prefer something that works efficently with the technology that's available to him. Something that is relatively cheap to produce and doesn't require any hand-wavium to construct.
What for? To destroy the main fort of the opposing forces. Or at least damage it beyond repair. There's just one tiny problem: It's mostly made of Iron and Stone.
- The fort stretches over an area of about 8km² and is surrounded entirely by walls that reach heights of 12m.
- The walls are made of stone and have a small core structure made of iron inside of them (so: stone-iron-stone, 6m-2m-6m).
- The buildings are mostly build with the same principle in mind. Build a small relatively small core of iron and "coat" it with stone.
- There are roughly 4.000 men living in this fort, all armed with melee- and ranged weaponry.
- The fort NEEDS to be destroyed, as it blocks a path to the Evil Overlords destination with no other way around it. Preferably in a matter of a couple years or less.
That should be enough (hopefully non-confusing) information. So the core question:
What weapon could my Evil Overlord build and use against this fort with the technology he has available, without using any hand-wavium and without magic?