Investment is the military tactic of surrounding an enemy
fort (or town) with armed forces to prevent entry or escape.
[1]A circumvallation is a line of fortifications, built by the attackers around the besieged fortification facing towards the enemy fort (to protect itself from sorties by its defenders and to enhance the blockade). The resulting fortifications are known as 'lines of circumvallation'. Lines of circumvallation 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 nearer to the city.
In cases where the besieging army is threatened by a field army allied to the enemy fort, the besieging army may construct a contravallation, a second line of fortifications outside the circumvallation, facing away from the enemy fort. The contravallation protects the besigers from attacks by allies of the city's defenders and enhances the blockade of the enemy fort by making it more difficult to smuggle in supplies.
The Siege of Alesia which took place in September 52 BC is one of the most famous investments in history. Julius Caesar in his Commentaries on the Gallic War describes his text book use of the circumvallation and contravallation to defeat the Gauls under their chieftain Vercingetorix.
