Use the Glass Eye from your backpack and the tiger’s mouth will open, revealing a key to the Armory.
How to Get the Glass Eye
You need to light up first all the four chandeliers in the Fire Place. There will be a lot of jumping and candle-lighting, but once you have all four chandeliers lit, the room will be bright and you can see everything inside. Now jump to the bottom and run to the left, past the fireplace and bookshelf. Walk a few steps to the left and you’ll find and pick up a Glass Eye that has fallen onto the floor.