Castles have played an important military, economic and social role in Great Britain and Ireland since their introduction following the Norman invasion of England in 1066. Although a small number of castles had been built in England in the 1050s, the Normans began to build motte and bailey and ringwork castles in large numbers to control their newly occupied territories in England and the Welsh Marches. During the 12th century the Normans began to build more castles in stone – with characteristic square keep – that played both military and political roles. Royal castles were used to control key towns and the economically important forests, while baronial castles were used by the Norman lords to control their widespread estates. David I invited Anglo-Norman lords into Scotland in the early 12th century to help him colonise and control areas of his kingdom such as Galloway; the new lords brought castle technologies with them and wooden castles began to be established over the south of the kingdom. Following the Norman invasion of Ireland in the 1170s, under Henry II, castles were established there too.

Dover Castle is a medieval castle in Dover, Kent, England and is Grade I listed. It was founded in the 11th century and has been described as the "Key to England" due to its defensive significance throughout history. Some writers say it is the largest castle in England, a title also claimed by Windsor Castle.