As a result, castle building by the Norman nobility across England and the Marches lacked a grand strategic plan, reflecting local circumstances such as military factors and the layout of existing estates and church lands. Castles were often situated along the old Roman roads that still formed the backbone for travel across the country, both to control the lines of communication and to ensure easy movement between different estates. Many castles were built close to inland river ports and those built on the coast were usually located at the mouths of rivers or in ports, Pevensey and Portchester being rare exceptions. Some groups of castles were located so as to be mutually reinforcing – for example the castles of Littledean Camp, Glasshouse Woods and Howle Hill Camp were intended to act as an integrated defence for the area around Gloucester and Gloucester Castle for Gloucester city itself, while Windsor was one of a ring of castles built around London, each approximately a day's march apart. Some regional patterns in castle building can also be seen – relatively few castles were built in East Anglia compared to the west of England or the Marches; this was probably due to the relatively settled and prosperous nature of the east of England and reflected a shortage of available serfs, or unfree labour.