The agger was an embankment that gave Roman roads the proper draining base. Basically the agger is a ridge that supports the road surface. The agger was constructed by excavating the line of the road, building a firm foundation, refilling and compressing the soil, adding more soil from digging drainage ditches or fosses on one or both sides of the road, then surfacing with graded layers of stone and cobbles. The material used to build the aggers was dug from lateral ditches. Once the material was dug out of the ditches that were known as " scoop ditches," they were used as the storm drain for that road. These ditches could also be used for soldiers to hide in if they ever were under attack from enemies.
On the most important road routes, the agger could be 4 to 5 feet (1.2 to 1.5 m) high and 45 to 50 feet (14 to 15 m) wide. Along less important routes the road is occasionally set directly on the levelled ground surface with stones laid to provide drainage with the lateral ditches barely visible. The material was usually found locally, though the Romans would not hesitate to bring in the material from other places if they could find no suitable stone. The course of a Roman road can often be traced today by the distinctive line of the agger across the landscape.
A well-known example is the Agger Servianus, a part of the Servian walls of Rome, which protected the city on its most vulnerable side, the Campus Esquilinus. It consisted of a double rampart bearing formidable fortifications.