AGGER, in the ancient military art, a work of fortification, used both for the defence and the attack of towns, camps, &c.; in which sense it is the same with what was otherwise called vallum, and in later times aggeratum; and, among the moderns, lines, sometimes cavaliers, terrasses, &c. The agger was usually a bank or elevation of earth or other matter, bound and supported with timber; having sometimes turrets on the top, wherein the workmen, engineers, and soldiery, were placed. It was also accompanied with a ditch, which served as its chief defence.
The height of the agger was frequently equal to that of the wall of the place. Cæsar tells us of one he made which was 30 feet high and 330 feet broad. Besides the use of aggers before towns, the generals used to fortify their camps with such works.