Complex sentence: It has commonly been accepted that Richborough was the landing site for the AD 43 invasion, owing to mid- 1st century AD ditches and fortifications on the site, assumed to be to protect their bridgehead and supply depot.
Split into simpler sentences: It has commonly been accepted that Richborough was the landing site for the AD 43 invasion. These ditches and fortifications are assumed to have been built to protect their bridgehead and supply depot.