The Fortress in Valor Mortis presents a difficult challenge, characterized by formidable enemies and decaying architecture. Accessing this area requires defeating a high-level Grenadier who guards its entrance, a task that is difficult even for an appropriately leveled player. Furthermore, the path beyond the Grenadier involves navigating a poison swamp that can lead to instant death if the poison status effect is not managed.
While it is possible to attempt to defeat the Grenadier early in the game, progress beyond the initial poison swamp will be impossible until later in the playthrough. The game’s progression dictates that players must complete the University section and acquire the Shield Transmutation. After this, the player is returned to the Abandoned Church, where Louis will assign the task of visiting the 'castle ruins'.
With the Shield Transmutation obtained, players can then safely traverse the poison swamp and enter the Fortress. This ruined castle environment is populated with various enemies, offering a significant area to explore.