Complex sentence: The defences and Celtic field systems there date back to the 7th century − 3rd century BCE, though they might mask earlier developments; the hillfort was occupied until approximately 100BC, though it is possible that it was reoccupied in the Roman and post-Roman periods.
Split into simpler sentences: The defences and Celtic field systems there date back to the 7th century − 3rd century BCE, though they might mask earlier developments. The hillfort was occupied until approximately 100BC, though it is possible that it was reoccupied in the Roman and post-Roman periods.