Complex sentence: There is no record of fortifications at the site prior to the Byzantine period; in 705 an Arab attack on the town was repelled, but its Christian population later abandoned the settlement and Sis became a fortified frontier post of the Abbasid's.
Split into simpler sentences: There is no record of fortifications at the site prior to the Byzantine period. In 705 an Arab attack on the town was repelled, but its Christian population later abandoned the settlement and Sis became a fortified frontier post of the Abbasid's.