Dolwyddelan Castle is the traditional birthplace of Llywelyn the Great (c. 1173), but it seems unlikely that the castle existed at this time. Archaeological excavations have revealed the remains of an older castle on a small rocky knoll, known as Tomen Castell, in the valley below Dolwyddelan, and it is more likely that this is the birthplace. Dolwyddelan Castle, which stands in a much more prominent position, was most probably built by Llwelyn the Great early in the 13th century.

The castle guarded one of the main routes through the mountains of Snowdonia. It consists of a rectangular keep and a small D-shaped courtyard that contains another rectangular tower, the west tower, which was added at a later date to provide more accommodation. The castle was captured by Edward I in 1283 and was garrisoned for a short time by the English.

By the 19th century the castle was a ruin, but the keep was restored by Lord Willoughby de Eresby in 1848-50. The distinctive battlements and projecting drains date from this restoration.