Harlech was begun during King Edward I's second campaign in north Wales. It was part of an "iron ring" of castles surrounding the coastal fringes of Snowdonia, eventually stretching from Flint around to Aberystwyth; a ring intended to prevent the region from ever again becoming a focal point of insurrection and a last bastion of resistance. Following the fall of the Welsh stronghold of Castell y Bere, King Edward's forces arrived at Harlech in April, 1283, and building work began almost immediately. Over the next six years an army of masons, quarriers, laborers and other craftsmen were busily engaged in construction. In 1286, with the work at its height, nearly 950 men were employed under the superintendence of Master James. The final result was a perfectly concentric castle, where one line of defenses is enclosed by another. Unfortunately, the outer wall is ruinous today and fails to convey the true 13th-century effect.
(http://www.castlewales.com/harlech.html)