The creation of the burhs was a well ordered programme which obviously required central planning coupled with expert knowledge of existing conditions.
It is likely that the construction of the burhs fell into three main phases:
The use of existing sites such as towns of Roman origin and Iron Age hillforts.
The creation of new towns; both promontory towns -where the existing topography provided part of the defence system, and new towns on open sites – where the defences had to be constructed totally ad novum. These towns were often situated at key points, such as river crossings.
“Filling in the gaps” by the creation of new forts, to ensure that no one in Wessex was more that a day’s march from a burh. Probably the burh of Southwark was also added at this time.