Immediately after the Norman Conquest, King William installed three of his most trusted allies, Hugh d'Avranches, Roger de Montgomerie, and William FitzOsbern, as Earls of Chester, Shrewsbury and Hereford respectively, with responsibilities for containing and subduing the Welsh.
The lands given to them were on the border with Wales also known as the March, and in exchange for their extensive lands and special privileges, the Marcher lords were expected to stop the Welsh from supporting English rebels in the North against King William.
The Marcher lords were very powerful and had the authority of a king in their lands. They did not have to pay tax on their lands and they were allowed to build towns and markets, which they were allowed to tax. In Norman England a noble needed permission from the king to build a castle, but the Marcher lords were exempt from this and they built hundreds of castles in 11th, 12th and 13th centuries.
Castles were powerful defensive structures but it was also the place which ordinary people associated with authority. They were important centres of administration and local government. Tax collectors, officers of the court and market traders could also be found within the walls of a castle. Because they were home to large garrisons, castles eventually became the centre of local activity.
The lord who controlled and maintained a castle was to some extent responsible for the security of the people in surrounding towns and villages, and the presence of a castle created a sense of security for the community. Their presence certainly helped maintain order in England throughout the Middle Ages.