At the start, go through the door, shoot the Nazi and move the picture. Behind the picture is a handle, that opens a secret room over the fireplace. Put a chair on the table, and move it as far as possible towards the fireplace. This will break several laws of nature. Climb the back of the chair, defy gravity, and jump to the top of the fireplace. Slash the flag and grab the gold.

...Who the heck invents something like this...?

Shoot both guards in the wine cellar. Between two of the barrels, a stone sticks a bit out. Press it, and a secret room opens.

Some wine is hidden here. Excellent vintage.

After some quite intense combat in the basement, go left inside the big room. Break the pinboard. Behind it, the wall look funny. Break the wall too, and you will find more gold.

Go through the closed door from the 'gold' room. At the end of the corridor, there is a door with a red light. Behind it, there is two generators and a barrel. Blow the barrel up (pls don't kill yourself) and it will break the wall to a secret room.