What defensive features were built along Hadrian’s Wall?

Hadrian’s Wall featured a sophisticated, multi-layered defensive system designed to control movement and counter threats along the northwestern frontier of Roman Britain. Its primary components included:12

• The Wall: A continuous barrier originally built of stone (10 feet wide in the east) or turf (20 feet wide in the west), fronted by a deep ditch on the northward side to impede approach.12
• Milecastles and Towers: Regularly spaced fortlets (milecastles) containing gates and barracks were positioned every mile, with two observation towers situated between each milecastle for patrolling.12
• Forts: Approximately 16 major forts were integrated into the wall line to house the fighting garrison, all connected by a rearward road.2
• The Vallum: A broad, flat-bottomed ditch located to the south of the wall, flanked by continuous earth mounds. It served as a rearward obstacle to delimit the military zone.12
• Coastal Defenses: The system of towers, fortlets, and forts extended at least 26 miles beyond the wall's western terminus along the Cumbrian coast to guard against incursions.12