Definition: Hadrian's wall was built, beginning in A.D. 122, to keep Roman Britain safe from hostile attacks from the Picts. It was the northernmost boundary of the Roman empire until early in the fifth century when the Atnonine Wall was built. Hadrian's Wall stretches from the North Sea to the Irish Sea (from the Tyne to the Solway). It was 80 Roman miles (about 73 modern miles) long, 8-10 feet wide, and 15 feet high. In addition to the wall, the Romans built a system of small forts called milecastles (housing garrisons of up to 60 men) every Roman mile along its entire length, with towers every 1/3 mile. Sixteen larger forts holding from 500 to 1000 troops were built into the wall, with large gates on the north face. To the south of the wall the Romans dug a wide ditch, (vallum), with six foot high earth banks.