The forces from Ulandus had taken as many aqueducts as they could, knowing that whoever controlled the water supply would be in the best position to continue their attacks. A small force of the Ulandans had decided that it would be faster to press-ganged some of the local farmers into hauling water for them to increase the flow instead of fixing the water-pump.
The men of Budd-apest, fighting for the Desert King, saw the slavery in action and decided to do something about it.
OoC: I originally wasn't going to build for this WZ. But then I got this idea in my head. I hope it works as a "medieval pumping station." The instructions just said to make it realistic based on what they had available back then. And what was more readily available than men?