Caradigan, Cardican, Ceredigion, Karadigan, Kardigan, Karidagan
According to Chrétien de Troyes, the stronghold of Cardigan was one of Arthur's places to hold court. Malory mentions Cardican in passing, as a castle where Sirs Percivale and Aglovale lodged for a brief time while searching for Lancelot.
In the tale of Meriadeuc, the queen of Cardigan is named Lore. Her city was besieged and captured by King Ris of Outre-Ombre, but she eventually forced him to leave. In the Livre d’Artus, it is ruled by Lisanor (Lyzianor) and is besieged by Saxons during the Saxon invasion at the beginning of Arthur’s reign.
From the name alone, I assume Malory's Cardican Castle to be Cardigan city, at the southwest of Cardiganshire, Wales; and would identify it with Chrétien's Cardigan.
Cardigan is a corruption of Ceredigion (Ceredigyawn), which is the original name, so called after King Ceredig (died 453), son of Cunedda Wledig. He became its king about the end of the fourth century.