C. Julius Caesar, Gallic War

Having arrived at Uxellodunum, contrary to the general expectation, and perceiving that the town was surrounded by the works, and that the enemy had no possible means of retiring from the assault, and being likewise informed by the deserters that the townsmen had abundance of corn, he endeavoured to prevent their getting water. A river divided the valley below, which almost surrounded the steep craggy mountain on which Uxellodunum was built. The nature of the ground prevented his turning the current: for it ran so low down at the foot of the mountain, that no drains could be sunk deep enough to draw it off in any direction. But the descent to it was so difficult, that if we made opposition, the besieged could neither come to the river nor retire up the precipice without hazard of their lives. Caesar perceiving the difficulty, disposed archers and slingers, and in some places, opposite to the easiest descents, placed engines, and attempted to hinder the townsmen from getting water at the river, which obliged them afterward to go all to one place to procure water.