Unit cohesion plays a key role in the success or failure of many kinds of military scenarios. On land or at sea, soldiers in the ancient world depended on one another to maintain battle lines and to keep up morale. But in siege warfare, there were many strands of loyalty pulling on the citizen-soldier. He had a duty to his family, his neighbours, his city, and his gods – and how he, and his comrades, balanced these priorities could mean the difference between victory and utter defeat. This paper will answer the question of how unit cohesion was maintained among the defending force during and in the run up to siege, using a combination of horizontal and vertical models (the commitment of individual soldiers to each other, and their commitment to the hierarchy and chain of command, respectively).The theory of how to create and maintain unit cohesion in a besieged city forms the basis of one of our earliest surviving military treatises, Aeneas Tacticus’ Poliorcetica. Aeneas describes systems to help ensure that key areas of the city are defended quickly and efficiently, how to prevent loss of morale among the soldiers, and how to create hierarchies within the city which would best promote cohesion. Homonoia, or unanimity, among both the military and the civilian populations is Aeneas Tacticus’ key to success. This paper will analyse Aeneas Tacticus’ advice unit cohesion alongside historical examples drawn from a range of sources covering the fifth and fourth centuries BC to examine the theoretical and practical ways in which Greek cities were able to create and maintain cohesion among soldiers defending besieged cities.