Castal I was the first planet in its star system, in the Alpha Quadrant.
In the 2350s, this planet was disputed between the Federation and the Talarians during the Galen border conflict. Talarian captain Endar lost his son during the battle at Castal I. (TNG: "Suddenly Human")
This planet was only mentioned in dialogue.
This planet's quadrant of origin is inferred based on the position of the Talarian Republic as seen in the star chart appearing in the Star Trek: Picard episode "Maps and Legends".