in return. The Doge, alarmed by Norman control of the Strait of Otranto, took command of the Venetian fleet and sailed at once, surprising the Norman fleet under the command of Bohemond as night was falling. The Normans counter-attacked tenaciously, but their inexperience in naval combat betrayed them. The experienced Venetian navy attacked in a close formation known as "sea harbour" and together with their use of Greek fire "bombs", the Norman line scattered, and the Venetian fleet sailed into Dyrrhachium's harbour. Siege of Dyrrhachium Robert was not discouraged by this naval defeat, and began his siege of Dyrrhachium. In command of the garrison at Dyrrhachium was the experienced general George Palaiologos, sent by Alexios with orders to hold out at all costs while Alexios himself mustered an army to relieve the city. Meanwhile, a Byzantine fleet arrived and – after joining with the Venetian fleet – attacked the Norman fleet, which was again routed. The garrison at Dyrrhachium managed to hold out all summer, despite Robert's catapults, ballistae and siege tower. The garrison made continuous sallies from the city; on one occasion, Palaiologos fought all day with an arrowhead in his skull. Another sally succeeded in destroying Robert's siege tower. Robert's camp was struck by disease; according to contemporary historian Anna Comnena up to 10,000 men died, including 500 knights. Even so, the situation of the Dyrrhachium garrison grew desperate because of the effects of Norman siege weapons. Alexios learned of this while he was in Salonica with his army so he advanced in full force against the Normans. According to Comnena, Alexios had about 20,000 men; historian John Haldon puts the army's size between 18,000 and 20,000 men, while John Birkenmeier estimates it between 20–25,000 men. It consisted of Thracian and Macedonian tagmata, which numbered about 5,000 men; the elite excubitors and vestiaritai units, which numbered around 1,000 men; a force of Manichaeans which comprised 2,800 men, Thessalian cavalry, Balkan conscripts, Armenian infantry and other light troops. As well as the native troops, the Byzantines were joined by 2,000 Turkish and 1,000 Frankish mercenaries, about 1,000 Varangians and 7,000 Turkish auxiliaries sent by the Seljuk Sultanate of Rûm. Alexios also withdrew the tagmas from Heraclea Pontica and the remaining Byzantine holdings in Asia Minor and by doing so, he effectively left them to be overrun by the Turks. Battle Initial moves Alexios advanced from Salonica and pitched camp