The size and importance of London led to the redevelopment of the city's defences. During the early medieval period – following the Norman Conquest of England – the walls underwent substantial work that included crenellations, additional gates and further towers and bastions. Aside from the seven City Wall gates and the four bars, there are the 13 water-gates on the Thames where goods were unloaded from ships. These include Billingsgate and Bridge Gate. Additionally there were pedestrian-only gates such as Tower Gate and the postern gate at the Tower of London.