Why did the Romans build forts behind and along Hadrian’s Wall?

The Romans built forts behind and along Hadrian’s Wall to serve as a strategic military system designed to control movement across the frontier and counter low-intensity threats from barbarian invaders.12

The placement and purpose of these fortifications evolved as the frontier policy developed:

• Control and Defense: The wall itself was not intended for fighting from the top; instead, the units stationed in the forts were trained and equipped to engage the enemy in the open. The forts provided a base for the auxiliary troops who manned the frontier.1
• Strategic Layers: The defensive works were constructed in layers. While the wall marked the limit of the Roman world, the forts—along with milecastles and towers—were integrated into a system connected by a road behind the wall to facilitate patrols and rapid response.2
• Rearward Obstacles: The vallum, a large ditch and mound system, was constructed behind the wall and its forts to serve as a rearward obstacle, effectively delimiting the military zone.12
• Historical Evolution: Early forts associated with the frontier were located behind the wall line on the Stanegate Road. As the Hadrianic scheme reached its final form, forts were integrated directly onto the wall line at roughly 7-mile intervals to enhance the efficiency of the frontier defenses.12