Built under the supervision of James of St George towards the end of the 13th century, Harlech Castle is one of the four great castles in Wales constructed for Edward I. Concentric in design, the rectangular inner bailey, with a round tower at each corner and a gatehouse mid way along the east wall, is surrounded by a second rectangular fortification, and the outer bailey enclosed by an outer curtain wall.

Situated on the top of a rocky elevation (about 200ft (60.6m) above the shoreline of Tremadoc Bay), this superb location provided Harlech Castle with naturally strong defences. The castle's only potential weakness was from the east and south, so a wide, deep ditch was cut into the rock surface to prevent attack from these points, and a gatehouse was built on the higher ground to the east.

This magnificent gatehouse is the most impressive, surviving structure within the inner bailey. Resembling a Keep (or great tower), inasmuch as it was a self-contained fort, the three-storey structure has two cylindrical towers flanking the entrance passage, and on either side lie two guardrooms. Two smaller, cylindrical towers project into the inner bailey. As with a traditional Keep, the gatehouse also contained domestic accommodation.

Little else in the inner bailey survives above foundation level, but the original domestic buildings can be identified as a chapel, bakehouse and well along the northern wall, a granary and the small hall along the southern wall, and the great hall along the west wall. The outer bailey closely surrounds the inner bailey and provides a good point from which to study the design and structure of the inner bailey walls. Harlech Castle withstood a Welsh attack at the very end of the 13th century, but was attacked again during the early 15th century and surrendered. Harlech Castle was regained by the English and once again came under royal control a few years later by the future King Henry V.

The last Royalist stronghold in Wales, Harlech Castle witnessed its end in the Civil War but did not experience the wilful destruction so prevalent at that time. Only constant weathering and the natural process of decay over the last seven centuries has left Harlech Castle much as it stands today, but there is sufficient remaining to fully appreciate the splendid military architecture employed in its design. A massive and powerful coastal defence, Harlech Castle still appears formidable, with its unyielding strength and dominance set against a backdrop of the magnificent Snowdonia mountain range.