The London Wall was a defensive wall first built by the Romans around the strategically important port town of Londinium in AD 200. It has origins as an initial mound wall and ditch from AD 100 and an initial fort, now called Cripplegate fort after the city gate that was positioned within its northern wall later on, built in 120-150 where it was then expanded upon by Roman builders into a city-wide defence. Over time, as Roman influence waned through the departure of the Roman army in 410, their withdrawal led to its disrepair, as political power on the island dispersed through the Heptarchy (seven kingdoms) period of Anglo-Saxon England. From the conquest of William the Conqueror, successive medieval restorations and repairs to its use have been undertaken. This wall largely defined the boundaries of the City of London until the later Middle Ages, when population rises and the development of towns around the city blurred the perimeter.