Back in the East Wing Corridor, head back towards the Main Hall. The first door on your left (which you passed earlier) leads to the Gallery. Enter it. Inside, you'll find several paintings. Examine the large painting at the far end of the room. This is a puzzle.
You need to press the buttons on the paintings in a specific order to reveal a hidden compartment. The order is: Young Man, Middle-Aged Man, Old Man, Infant, Old Man, Middle-Aged Man, Young Man. If you make a mistake, a zombie will drop from the ceiling. Once correctly entered, the large painting will slide away, revealing a hidden passage. Enter the passage.
Follow the short passage to a small room. On a pedestal, you'll find the Armor Key. Pick it up. Be aware that picking up the key will trigger a trap: the walls will start to close in. Quickly examine the small statue in the corner of the room and push it. This will stop the trap. Exit the room and the Gallery.