Roman bicircumvallation. A circumvallation is a line of fortifications, built by the attackers around the besieged fortification facing towards the enemy fort. The resulting fortifications are known as 'lines of circumvallation'. They generally consist of earthen ramparts and entrenchments that encircle the besieged city. The line of circumvallation can be used as a base for launching assaults against the besieged city or for constructing further earthworks closer to the city.