Complex sentence: In order to secure his authority during the civil war Cantacuzenus hired Turkish mercenaries, these mercenaries although they were of some use, in 1352 they seized Gallipoli from the Byzantines.
Split into simpler sentences: In order to secure his authority during the civil war, Cantacuzenus hired Turkish mercenaries. Although these mercenaries were of some use, in 1352 they seized Gallipoli from the Byzantines.