Ballista - Caesar covered his landing in Britain with fire from catapults and ballistas.
The ballista had horizontal arms like a bow. The arms were set in rope; a cord, fastened to the arms like a bowstring, fired arrows, darts, and stones. Like a modern field gun, the ballista shot low and directly toward the enemy.
Catapult
The catapult was the howitzer, or mortar, of its day and could throw a hundred-pound stone 600 yards in a high arc to strike the enemy behind his wall or batter down his defenses. "In the middle of the ropes a wooden arm rises like a chariot pole," wrote the historian Marcellinus. "At the top of the arm hangs a sling. When battle is commenced, a round stone is set in the sling. Four soldiers on each side of the engine wind the arm down until it is almost level with the ground. When the arm is set free, it springs up and hurls the stone forth from its sling." In early times the weapon was called a "scorpion," for like this dreaded insect it bore its "sting" erect.
trebuchet
The trebuchet was another war machine used extensively during the Middle Ages. Essentially, it was a seesaw. Weights on the short arm swung the long throwing arm.