Harlech Castle, located in Harlech, Gwynedd, Wales, is a concentric castle, constructed atop a cliff beside the Irish Sea. Built by King Edward I during his conquest of Wales, the castle was subject to several assaults and sieges during its period of active use as a fortification. The seven-year siege of the castle has been memorialised in the famous song, "Men of Harlech". Edward I's second Welsh campaign, the castle was part of Edward's "iron ring" of castles around Snowdonia, "a string of new castles to hem the prince in". Construction began in 1283, "within days of Edward's arrival".Like many of the castles in the area, Harlech was designed by Master James of St. George. The castle took seven years to build, and cost an estimated £8,190 to build. Following its completion, James was appointed Constable of Harlech Castle, a position he held for over three years. All the royal castles of Edward's second Welsh campaign were sited "so that they could be kept supplied at all times".

The castle is built to a concentric plan, with one line of defences enclosed by another. The outer walls are much shorter and thinner than the mighty inner walls, and have no towers defending them besides the small gatehouse. The inner ward is roughly square, with a large round tower at each corner. The domestic buildings, including the great hall, are built against the inside of the inner walls. Since the surrounding cliffs made it practically impossible to attack the castle except from the east, this side is faced by the imposing gatehouse. The gate is flanked by two massive "D-shaped" towers, the standard plan of the era, and defended by a series of doors, portcullises and murder-holes. Noteably, there are large windows on the inner face of the gatehouse, showing its second role as the premier domestic accommodation. The west wall of the inner ward also has large windows (as it forms one wall of the great hall), which would make it vulnerable were it not for the aforementioned cliffs. The outer ditches at Harlech were "hacked through solid rock.