Complex sentence: Although most of the visible structure is older, the course of the Roman wall was used for Exeter's subsequent city walls, meaning about 70% of it remains and most of its route can be traced on foot.
Split into simpler sentences: Although most of the visible structure is older, the course of the Roman wall was used for Exeter's subsequent city walls. Thus about 70% of the Roman wall remains, and most of its route can be traced on foot.