The oppidum of Alesia besieged in 52 BC by the Roman legions of Julius Caesar was almost entirely surrounded by a fourteen-mile (21 km) long trench filled with water from the diversion of a local river. Next came another trench and then another wooden palisade reinforced with watchtowers and attack towers. The besieging Roman army was entrenched behind another series of palisades and ditches to protect against the Gallic reinforcements called in by Vercingetorix. In keeping with the siege techniques of the time, ballistae transported with the legions and assembled on site were assigned to snipers. They were installed on the large siege towers or directly on the battlefield. As for the troops, they were armed with bows and slings.