Excerpts from Books and Wikipedia
The Roman Empire reached its greatest extent under Emperor Trajan (r.97-117). Hadrian became emperor when Trajan died. Hadrian thought the empire was too large to govern. He abandoned Dacia, north of the Danube, and pulled the eastern frontier back to the Euphrates. Hadrian's Wall was 80 miles (128 km) long in northern England. The stone wall included regularly placed look-out posts and garrison forts. It took about 10 years to build. [Furtado: 1001 Days]