The Romans were master engineers and builders. One of the most famous examples of this is when Caesar built two encircling walls around Alesia, one to besiege the city and the other to protect his army from attacks in the flank. Legions on the march were able to create fortifications every night, establishing temporary fortresses in hostile territory. This allowed the legions to sleep at night knowing they were protected from night time raids. It also allowed for the legion to retreat when defeated in battle to their fortified encampment meaning that defeats were not as severe.

I've constructed a segment of a Roman fortification. It contains palisades which allow legionnaires on the walls to fire down javelins and other missiles upon attackers. In front of the palisades is a ditch and moat, this means that any siege works which approach the walls will get bogged down and stuck in the ditch or flooded in the moat. There are also stakes places throughout the ditch, this greatly decreases the effectiveness of a charge by enemy soldiers and slows them down allowing the legionnaires on the wall to attack them easier. The Romans also cleared the trees and bushes surrounding the encampment so that the men on the walls could spot the enemies before they were on top of them.