Complex sentence: It is from the time of the Roman Empire that many of the archaeological finds of ballistae date and in these times that many of authors wrote their accounts and whose technical manuals and journals are used today by archaeologists to reconstruct these weapons.
Split into simpler sentences: It is from the time of the Roman Empire that many of the archaeological finds of ballistae date. Accounts by the finders, including technical manuals and journals, are used today by archaeologists to reconstruct these weapons.