The Plains of Denaria are a wasteland characterised by a rocky, dusty and barren landscape. It's unknown which part of the land of Albion the Plains are located, but it's likely that they are somewhere between the city of Camelot or The Valley of the Fallen Kings (since Morgana's Hovel, where she took care of Morgause, is likely to be there) and it is close to the Isle of the Blessed.
When war broke out between Uther and Caerleon, their armies met and fought on the Plains of Denaria in a battle that is known as the Battle of Denaria. When Carleon was about to be defeated, he retreated to the Castle of Fyrien. This indicates that the Plains are also near the Castle of Fyrien and, thus, near The Great Seas of Meredor (this is very likely, as the Isle of the Blessed is near the Seas of Meredor) (The Castle of Fyrien).
A year after, Morgana and an injured Morgause left Camelot; Sir Leon, Sir Elyan, Sir Bertrand, and Sir Montague caught up with Morgana on the Plains of Denaria when she was taking her sister to the Isle of the Blessed. When they approached her, she used stunning spells to knock them out. While Elyan and Leon survived, Sir Bertrand and Sir Montague unfortunately did not (The Darkest Hour).
- The Plains of Denaria were used in the first scene of Series 4.