When Edward I conquered Wales, he did more than build castles. He also built townships. These were villages associated with one of his castles. In most cases, he imported English people to live in them, ousting the native Welsh. Caernarfon, Rhuddlan, Conwy, Flint, Harlech and Beaumaris were among these combined castles/villages.
“The strategy of building Welsh Medieval Castles was combined with King Edward’s ambition to build and integrate fortified towns with the great castles. These purpose-built townships were designed to predominantly house the English conquerors. The towns were defended by the city walls and, of course, the castles. The Constable of the castle would often perform a dual role as Mayor of the town. Not only did the English have control over the local Welsh population they also had control of commerce and finance. The townships were established as trading centres – for the English. Welshmen were generally forbidden from entering the townships, let alone conducting trade in them.” http://www.ancientfortresses.org/welsh-fortified-towns.htm
From UNESCO: “The royal castles bear unique testimony of construction in the Middle Ages. The accounts that have survived specify the origin of the workmen, who were brought in from all regions of England, and describe the use of quarried stone on the site. They outline financing of the construction works and provide an understanding of the daily life of the workmen and population and thus constitute one of the major references of medieval history.
Throughout his reign (1272-1307) Edward I, King of England, worked to expand and defend his domain, implementing at the same time a military and settlement policy whose traces are still visible from the Pyrenees to Scotland. Above all in Wales, it is the major illustration of the great construction policy of his reign: a series of superb castles, which in some cases are combined with new towns surrounded by fortified walls, are the examples of the medieval urban planning.
From 1283 he undertook a castle-building programme of unprecedented scale. What he did was to station garrisons so as to quell any possible revolts, foster the settlement of castral towns by settlers and finally illustrate in a more symbolic than strategic fashion English power.” http://whc.unesco.org/en/list/374