Great Stones of Nemeton

"Each year at Beltane, the High Priestesses would gather at the Great Stones of Nemeton and summon the spirits of their ancestors."

- Gaius to Arthur

The Great Stones of Nemeton are a ring of stone structures located in the kingdom of Camelot. In the time of the Old Religion, High Priestesses would gather at the Stones to summon the spirits of their ancestors. As such, they are considered sacred by followers of the Old Religion.

History
According to Gaius, who had taken parts in such ceremonies long before the time of the Great Purge, each year at Beltane the High Priestesses would gather at the Great Stones of Nemeton and use the Horn of Cathbhadh to summon the spirits of their ancestors.

On the third anniversary of his father's death, Arthur rode to the Great Stones of Nemeton and used the Horn of Cathbhadh to summon Uther's spirit from the Spirit World (The Death Song of Uther Pendragon).

Trivia

 * The Great Stones of Nemeton are similar in appearance to Stonehenge, a prehistoric monument located in Wiltshire, England. However, the structure depicted in Merlin has fewer stones, most of which are made with visual effects.

Historicity
Nemetons were places of worship of ancient Celtic religion. They were primarily situated in natural areas such as sacred groves, but could also be located in spaces such as shrines and temples.