Din Eidyn, Mount Eidyn
A Welsh poem notes that Arthur’s warriors fought against Dogsheads on the mount of Eidyn.
The Mount of Eidyn is believed to be an ancient name for what is now known as Castle Rock, a volcanic rock formation in the heart of Edinburgh, Scotland. In early medieval times, the area around Castle Rock was known as Din Eidyn, which is thought to have been a stronghold of the Gododdin, a Celtic-speaking people.
In the poem Y Gododdin, Din Eidyn is described as the court of the ruler Mynyddog Mwynfawr, and it is mentioned in the context of a tragic battle in the early sixth century. The Gododdin warriors, including King Mynyddog’s son, engaged in a battle against the Angles at Catraeth (possibly modern-day Catterick in North Yorkshire, England). The battle resulted in heavy losses for the Gododdin, and it is considered a significant event in early British history.
Over time, the name Din Eidyn evolved into Edinburgh, and the hill itself became known as Castle Rock due to the presence of Edinburgh Castle atop the volcanic formation.
Source
Pa gur yv y porthaur | Poem 31 of the Black Book of Carmarthen, probably c. 1100