It was the northernmost frontier barrier of the Roman Empire . It spanned about 39 miles ( 63 km ) , and was about ten feet ( 3 m ) high and fifteen feet ( 5 m ) wide . Representing the northernmost frontier barrier of the Roman Empire , it spanned approximately 39 miles ( 63 km ) and was about ten feet ( 3 m ) high and fifteen feet ( 5 m ) wide .