Complex sentence: Ad Pirum was a fortress during the time of the late Roman Empire representing the center of the Claustra Alpium Iuliarum, a network of forts and wall to secure Italia from eastern invasions, including the road between Italia and Pannonia.
Split into simpler sentences: Ad Pirum was a Roman fortress active during the time of the late Roman Empire. This structure represented the center of the Claustra Alpium Iuliarum, a network of forts and walls to securing Italia from eastern invasions, including the road between Italia and Pannonia.