Strange things happening at a siege

[Gateway103]

However, most likely the 1400 units aren't supply-freem so you can starve them out, even if they are repairing the wall faster than you can break it. Once their number dwindles due to starvation, sieging may start to take effect.

There is also an Earth spell capable of reducing castle defense. Not exactly going to help you though, since magic phase occurs first, so the wall would be damaged or destroyed, but then he'll repair some back if not fully, so by the start of next time it still has partial defense.

At the very least, you can hole up his troops inside the fort, while you send a second army to conquer the surroundings, and then preach him out of existence.

-Gateway103