The Old castle ruins (旧城,Kyūjōseki?) is an area of the underground system underneath the castle. Away from the spectacular fortress that was Salazar's castle, the remains of a separate, older building were largely abandoned save for Los Iluminados cult's mining of the area underneath (which included Underground ruins) in search of any remaining Plaga parasites.
Ada's note can be found inside a shack where a merchant and typewriter are also available. A group of villagers appears to be standing around a huge fire. The Stone of Sacrifice is required to enter the Cathedral , where it can be found inside the underground ruins.
There is a unique Ambient AI in this area. On the lone tree before the any of the ruins there is a black beetle on it. Causing any damage to it will make it fly off in a set path toward the sky. After the initial hit, it can not be hit or interacted with again. It does respond by exiting and reentering the area.