Aeneas Tacticus 22.1-8: Preventing communication with the enemy.

1. Watches at night must be strictly kept in time of war and when the enemy are close to the city or camp. 2. The commander-in-chief and his bodyguard should be stationed round the town hall and market place, if this position is a defensible one, otherwise, he should have previously occupied the strongest place in the city, and the most conspicuous from all quarters. 3. The bugler and the dispatch-runners should always be quartered next to the general’s lodging, ready at hand in case bugle-calls or messages are needed, so as to give the guards and the rounds notice of what is to be done, wherever they happen to be in their circuit of the city. 4. Secondly, the guards on the wall, in the market place and at the town hall, the entrances to the market place, the theatre, and other points occupied should have short periods on duty: the reliefs should be frequent and their numbers strong. 5. For in a short period on guard a man will not have time to effect communication with the enemy and complete any treasonable design before he is relieved, and men will be less likely to fall asleep at their posts if they are on duty for a short time only; and with large numbers it is more likely that information will leak out concerning any attempt at treachery. 5a. Thus it is desirable that as many men as possible should be on the alert at time of danger, and that everyone should go on guard duty during the night, so that there may be as many men as possible in each relief; 6. with small numbers and infrequent reliefs men are likely to fall asleep owing to the length of their watch, and intending traitors will have ample time to communicate with the enemy unobserved before they are relieved. These considerations, therefore, have to be borne in mind.
7. At a critical time these further precautions should be added. None of the sentinels should know beforehand in which relief or at what point in the city he will be on guard; nor should the same commanders be always in charge of the same detachment; in all matters concerned with the supervision of citizens changes should be made as frequently as possible. A traitor will have far less chance of betraying anything to outsiders or receiving information from the enemy, 8. when no-one knows beforehand at what point of the wall he will be at night, or who his companions will be, but everyone is in complete ignorance of his destination. Those who have kept guard by day should not do so at night as well: for it is inadvisable that men should know in advance on what duties they will be deployed.