Complex sentence: Gülek citizens believe that the ancient inscriptions in the pass were erected by Alexander; actually, these inscriptions were erected much later by the Roman Emperor Caracalla (reigned 211--217) during his campaign against the Parthian Empire.
Split into simpler sentences: Gülek citizens believe that the ancient inscriptions in the pass were erected by Alexander. But, actually, these inscriptions were erected much later by the Roman Emperor Caracalla (reigned 211--217) during his campaign against the Parthian Empire.