Castiana (Taoth Vaclarush in Ancient) is one of the locations mentioned in Merlin's prophecy, being where King Arthur and his knights embarked on a quest to seek out the Sangraal, also known as the Bloodstone. The land is a great distance from Camelot.
Meurik told Lt. Colonel Cameron Mitchell and Dr. Daniel Jackson of Arthur's travels to Castiana, as well as Sahal and Vagonbrei, when SG-1 came in search of the Sangraal. Its exact location was revealed to Jackson by Morgan Le Fay in Atlantis; however the planet possessed a toxic atmosphere that made it difficult to explore. (SG1: "Camelot", "The Pegasus Project", "Insiders")
In fact, Castiana itself probably contained nothing of value, but its location within the galaxy was a clue. SG-1 ultimately used Stargate glyphs from Castiana's address, along with those of Sahal and Vagonbrei, to determine the location of yet another planet, which ultimately led them to discover Merlin's library. (SG1: "The Quest, Part 1")