Complex sentence: The many Roman forts of which ruins survive in Britain differed in being wholly military in nature, camps or strongholds of the Roman army: the Romans also built town or city walls in England which can still be seen, for instance at Silchester.
Split into simpler sentences: The many Roman forts in Britain that survive as ruins differed in being wholly military in nature, as camps or strongholds of the Roman army. The Romans also built town or city walls in England that survive, for instance, at Silchester.