Complex sentence: The fort was built during the 3rd century to protect the southern coastline of Britain from possibly Saxon raiders (part of the Saxon Shore) and occupies a commanding position at the head of Portsmouth Harbour.
Split into simpler sentences: The fort was built during the 3rd century as part of the Saxon Shore Forts to protect the southern coastline of Britain, possibly from Saxon raiders. It occupies a commanding position at the head of Portsmouth Harbour.