When Edward I finally pacified Wales, he could not simply go home and expect the Welsh to meekly behave themselves. Even the imposition of new laws and enforcing officers would not be enough. He needed strong bases from which he could operate in case of any further uprisings, so he embarked on a huge programme of castle building.
In South Wales where the Marcher Lords ruled there were already strong, stone-built castles at Caerphilly, Cardiff, Pembroke, Cardigan and Carmarthen. He also took over the castles of the former Welsh princes at Dinefwr and Dryslwyn. But in the north, where Llewellyn had reigned, he had to start from scratch. He built huge stone castles at Rhuthun, Denbigh, Holt and Hope, all started by 1282, followed by Conwy, Harlech and Caernarfon. In 1295, after another uprising in the north, he began Beaumaris castle on Anglesey.
The cost was immense. Most of the castles were finished by 1301, by which time the cost was over £80,000. Today that translates to about £60 million. Materials like stone, lead and iron, and craftsmen, were brought to North Wales from all over Britain.
Not only were the castles themselves a deterrent to uprising, but the Welsh were not allowed to live or work anywhere near them. They were run by Englishmen, who brought their own laws and their own men to run them and govern the area. This created a marked feeling of inferiority in the Welsh and superiority in the English that changed the face of the country, and persisted for centuries.
For photos of Welsh castles, see here.