Above is a model of one of Caesar’s larger redoubts. These like the smaller redoubts had towers as well as the normal multiple defense works.
In addition to the defenses the Romans had catapults placed at regular intervals to fire from behind or along the ramparts. The Romans had range markers to know when to start shooting.
Caesar was hard pressed to meet attacks from so many places. Once the Gauls inside Alesia saw the ensuing fight beneath their town they came out to help-all too late.