Construction on the wall started in 122 A.D. The wall, stretched from the North Sea to the Irish Sea and was about 73 miles long, 8-10 feet wide, and 15 feet high. In addition to the wall, the Romans built a system of small forts called mile castles which housed about 60 Roman soldiers each. The forts were built every Roman mile (about 80 total) 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.