Another of Edward I's massive and threatening fortresses, built to hold down a turbulent Wales, Harlech's castle is perched dramatically on a rock bluff high over what was once a tidal creek, with a moat defending it from high ground to the south and east. It is unusual in its plan, in that its strongest part is its huge gatehouse, instead of the usual massive central keep.

The castle's commanding position enabled it to be held by a garrison of only 37 men during the rebellion of Madoc ap Llywelyn in 1294. The defenders were supplied by sea from Ireland, the supplies being brought up a staircase, which still climbs the west side of the 200 ft castle rock today. In 1404, however, Owain Glyndwr captured the castle and made it the residence of his court until it was retaken by the English five years later.

Harlech Castle played a prominent part in the Wars of the Roses, when it was held by the Lancastrians. Its siege, which ended in surrender, is commemorated in the song Men of Harlech'. The castle was in ruins by the time of the Tudors, only the Prison Tower being left intact and used as a debtors' prison.