Harlech Castle is one of the great castles Edward I built to enforce his rule over the Welsh. Situated high upon a rocky outcrop, its seaward side was defended by sheer cliffs, while a deep moat protected the other sides. It was designed by Master James of St George, who personally supervised its construction, ensuring that the vast fortress was completed in just seven years (1283-1290). The castle is built to a concentric design with an impressive inner curtain wall with huge round towers on the corners, surrounded by an outer perimeter of much lower walls. The inner walls contained domestic buildings, including a great hall, and the more vulnerable east side of the castle was strengthened by a massive gatehouse that contained comfortable residential quarters.

Harlech Castle played a key role in the last great Welsh uprising, led by Owain Glyn Dwr. In 1404, after a long siege, the castle fell to Glyn Dwr, and became his home and headquarters for the next four years. Harlech was finally retaken by the English in 1409, under the command of Harry of Monmouth, prince of Wales - the future King Henry V. Although Glyn Dwr escaped, his family were captured, and the fall of Harlech marked the beginning of the end of the great uprising.

During the Civil War (1642-48), Harlech was the last Royalist stronghold to be lost, its fall signifying the end of the war. Like many other Royalist strongholds the castle was rendered untenable by the victorious Parliamentarians.