Complex sentence: The northernmost frontier barrier of the Roman Empire, it ran approximately 39 miles (63 km) and was of about ten feet high and fifteen feet wide; security was bolstered by a deep ditch on the north side.
Split into simpler sentences: The northernmost frontier barrier of the Roman Empire, it ran approximately 39 miles (63 km) and was about ten feet high and fifteen feet wide. Security was bolstered by a deep ditch on the north side.