Complex sentence: At the time of being destroyed by Rome, these villages must have still been in a pre-urban phase, beginning to group around a centre which may well have been Castel Gandolfo, whose significantly larger necropolis suggests a larger town.
Split into simpler sentences: At the time of being destroyed by Rome, these villages must have still been in a pre-urban phase, beginning to group around a centre. That center may well have been Castel Gandolfo, since its significantly larger necropolis suggests a larger town.