Cantre'r Gwaelod was a city ruled as part of the Kingdom of Meirionnydd by Gwyddno Garanhir (Longshanks), who was born about 520 AD.
The land was said to be extremely fertile, so much so that it was said that any acre there was worth four acres elsewhere. The catch was that the land depended on a dyke to protect it from the sea. The dyke had sluice gates that were opened at low tide to drain the water from the land, and closed as the tide returned.
In around 585 AD Seithennin, who was a visiting local King, was intent on amorously distracting the fair maiden Mererid, who was in charge of the sluice gates. The great and the good were celebrating the wedding of King Gwyddno's son in the great hall within the city where many a noble presented their Wedding gifts to the couple, leaving just Mererid to attend to the sluice gates.
A great storm was brewining up at sea at that time and distracted by their infatuation neither noticed what was happening. Successful in his amorous mission, Mererid was therefore too late to warn the city and was unable to shut the gates. The land was flooded as the warning bells sounded.
Whilst Seithennin and Mererid both escaped from the city before it was swallowed by the sea the rest of the city drowned in the ensuing flood including the King and his Swineherd Gethryn, rival to Gwydion. The city and its glories were lost and only the Poem written down in the Black Book of Camarthan survives to tell the tale of the loss.
