King Benlli was a British king who ruled part of what is now Wales

Wales

Wales is a country that is part of the United Kingdom and the island of Great Britain, bordered by England to its east and the Atlantic Ocean and Irish Sea to its west. It has a population of three million, and a total area of 20,779 km²...

 in the early 5th century. He is notorious for opposing Saint Germanus

Germanus of Auxerre

Germanus of Auxerre was a bishop of Auxerre in Gaul. He is a saint in both the Roman Catholic and Orthodox churches, commemorated on July 31. He visited Britain in around 429 and the records of this visit provide valuable information on the state of post-Roman British society...

 and was probably a heretical follower of Arianism

Arianism

Arianism is the theological teaching attributed to Arius , a Christian presbyter from Alexandria, Egypt, concerning the relationship of the entities of the Trinity and the precise nature of the Son of God as being a subordinate entity to God the Father...

. The story of his admonishment by the saint and eventual demise by "fire from heaven" is recorded in Historia Brittonum, chapters 32-35. The hill fort

Hill fort

A hill fort is a type of earthworks used as a fortified refuge or defended settlement, located to exploit a rise in elevation for defensive advantage. They are typically European and of the Bronze and Iron Ages. Some were used in the post-Roman period...

 at Foel Fenlli

Foel Fenlli

Foel Fenlli or Moel Fenlli is a hill in Denbighshire, North Wales. With a summit at an elevation of , it is the second highest peak of the Clwydian Range....

is traditionally considered to have been his castle.