"Screenshot from game: Ludus Mortis
Map: Catacomb of Abdon and Sennen
Secrets found: 4/4

1: Two Chests hiding behind a crushable wall
2: An invisible wall prevents access to a narrow area with a bag of silver and a switch for a nearby door
3: Invisible wall just west of the entrance to an altar
4: Crushable wall in the very west of the map

The area behind the door in the south of the map is requires a Purphuros key, which is only obtainable in a later dungeon
"