The siege of Cyzicus (73 B.C.) was a Roman victory that effectively ended Mithridates VI's campaign in western Asia Minor at the start of the Third Mithridatic War. Mithridates had begun with war with a victory over the Roman consul Cotta at Chalcedon, before moving west to besiege the port of Cyzicus, on the Asian shore of the Propontis.
Cyzicus was situated at the southern tip of an island facing towards the Asian shore of the Propontis. At the time of the siege the island was connected to the mainland by a single narrow causeway. Behind the city (on the island) were mountains named Arctonoros and Dindymus (Strabo), while across the narrow straits was another mountain, Adrasteia, and a suburb of the city.
Mithridates originally took up a position on the slopes of Adrasteia, before moving most of army over to the island. While his army attacked the city walls, his fleet blocked the straits, preventing supplies or news from reaching the city. The city had provided ships and troops to Cotta, and according to Plutarch had lost 3,000 men and ten ships during the battle of Chalcedon. Despite this, and despite the massive size of the Pontic army, the citizens of Cyzicus held out for long enough for Cotta's colleague Lucullus to arrive on the scene.
When Lucullus arrived at Cyzicus he decided that Mithridates's army was too large to attack, but also that its great size meant that Mithridates would soon run short of food. He built his first camp at the village of Thracia, from where he could prevent Mithridates from gathering any supplies in the surrounding countryside. Lucullus would then appear to have moved onto Mt. Adrasteia, overlooking the straits.
According to Appian Lucullus was able to capture this strongpoint because the Roman renegade Lucius Magius decided to change sides after hearing of the death of Sertorius. He convinced Mithridates that the two Fimbrian legions were willing to desert, and that the Romans should be allowed to take up whatever position they wanted, as they could be defeated without a battle. Unfortunately for this tale Sertorius was not murdered until 72 B.C., the year after the end of the siege (or two years if the siege is dated to 74 B.C.)
Although Lucullus's camp was now in plain view from Cyzicus, Mithridates is said to have convinced the besieged citizens that the Roman army was actually a force of Armenians and Medes sent by Tigranes of Armenia. Only when Lucullus managed to slip a messenger into the city did the defenders realise that the Romans had arrived.
After Lucullus arrived Mithridates focused all of his efforts on the siege of Cyzicus. If he had taken the city, then his fleet would have been able to keep him supplied, and Lucullus may found himself in a difficult position, but the city held out until the start of winter.
Mithridates built an impressive range of siege engines, including a tower 50 cubits (roughly 100 meters) high, topped with catapults, and a second tower that was carried between two quinqueremes that was designed to be using as a flying bridge to allow his troops to reach the top of the walls (similar to the sambuca used at the siege of Rhodes during the First Mithridatic War). This second machine reached the walls, but only four attackers crossed the bridge before the defenders poured burning pitch onto the two ships, forcing them to pull back. Mithridates also attempted to dig tunnels to undermine walls, but the defenders responded with counter-tunnels. A number of siege engines destroyed in a storm. Finally Mithridates attempted to build a mound between Mt. Dindymus and the city, and built siege engines on this mound, but the defenders of the city undermined the entire structure.
By this time winter was approaching. Famine already threatened the Pontic army, and at about the same time as building the mound Mithridates had sent away his cavalry and the injured and sick. This force had been intercepted and defeated by Lucullus at the River Rhyndacis.
After the collapse of the mound Mithridates decided to abandon the siege. He escaped by sea, while the infantry was left to make its way west along the coast towards the port of Lampsacus. Lucullus gave chase, inflicting heavy casualties on the retreating Pontic army at the crossings of the Aesupus and Granicus rivers. Of the original horde only a small fraction survived to reach Lampsacus, where the Roman renegade Marcus Varius took command. Although Mithridates attempted to continue his campaign around the Propontis, the virtual destruction of his army at Cyzicus soon forced him to retreat back into Pontus.