Richborough is believed to be the landing point of the Roman invasion armies in 43 AD. The foundations of a quadrifrons triumphal arch thought to commemorate this are in the middle of the earthworks shown in first photo.  The earthworks themselves were added in the third century when the arch became a coastal signal tower watching for pirates.  The huge stone walls were added in the late third century when the site became what is known as a Saxon Shore Fort, guarding the coast against marauders.  After the fall of the Roman Empire, an early Christian site was established within the abandoned fort walls.