Why did the Romans build defenses along the eastern border of the Agri Decumates?

The Romans annexed the Agri Decumates—a region located between the middle Rhine and upper Danube rivers—primarily to secure and improve communications between the Roman armies already stationed along those two major rivers.12

To protect this territory, the Romans established a fortified frontier known as the limes along its eastern border. This system, which reached its final form under the emperor Antoninus Pius, consisted of a continuous barrier of ditches and palisades, later reinforced with stone and earthen walls. These defenses were essential for maintaining control over the region and managing the frontier against external pressures.23