Cantre’r Gwaelod is the supposedly mythical land that lay within Cardigan Bay in pre-history, but was lost beneath the waves during a great storm. Causeways running out to sea for miles near Harlech and the fossil forest revealed at low tide at Borth are the only obvious relics still remaining. But hidden by mist and drizzle, and absent from mariners charts, the last islets still survive. A walled town, and port and a few fields and a small but independent nation.

Three issues have been identified. The first issue featured a simple Celtic square or diamond design in colours and two values. These were roulette perforated. The second issue was a larger diamond shape, also rouletted, in shades of green and brown and featuring a circular Celtic animal design. The pictorial stamps are multicoloured and feature an aerial impression of the town, and a harbour scene. These have coarse perforations. Values are I for internal mail, and II for mail to the mainland; no currency is given.