There are 595 Doors inside that lead to various whens and wheres. These doors were made by the Old Ones. They lead to important events in history, usually grim ones like Lincoln's assassination or the gladiator fights in Ancient Rome. Other Doors open upon the todash darkness. One of these todash monsters came through and attempted to eat Roland Deschain and Susannah Dean. They were able to hold the creature at bay, using cans of Sterno to light torches.
Castle Discordia was first seen in Wolves of the Calla. Mia imagines being in the castle as she is scavenging for food to feed her "chap". Later, the castle is seen as Mia and Susannah hold palaver about each other's needs.