The rolling siege tower was the big offensive weapon throughout the age of the castle. Helepolis means “Taker of Cities” and was rolled up to the alls of a fortress. It was 50 feet square at the base and over 100 feet tall. Inside the moving staircase archers, peltasts could fire into the defenders or infantry could wait until their time to enter the castle.
From Wikipedia: “The Helepolis bore a fearsome complement of heavy armaments, with two 180-pound (82 kg) catapults, and one 60-pounder (27 kg) (classified by the weight of the projectiles they threw) on the first floor, three 60-pounders (27 kg) on the second, and two 30-pounders (14 kg) on each of the next five floors. Apertures, shielded by mechanically adjustable shutters, lined with skins stuffed with wool and seaweed to render them fireproof, pierced the forward wall of the tower for firing the missile weapons. On each of the top two floors, soldiers could use two light dart throwers to easily clear the walls of defenders. As the Helepolis was pushed towards the city, the Rhodians managed to dislodge some of the metal plates, and Demetrius ordered it withdrawn from battle to protect it from being burned. Following the failure of the siege, the Helepolis along with the other siege engines were abandoned, and the people of Rhodes melted down their metal plating and sold abandoned weapons using the materials and money to build a statue of their patron god, Helios, the Colossus of Rhodes, known as one of the ancient Seven Wonders of the World.”
The Oxybeles was basically a gastraphetes on stand. It had a tremendous hitting power, but took two people to operate. These siege weapons go back to the 4th century BC.