That song in German sung in the castle...

... is none other than "Panzerlied", the famous anthem sung by Wermacht tank crews and made popular in a scene in BATTLE OF THE BULGE (1965). It is, by the way, still sang by tank crews in the current German armed forces (Bundeswehr). Why would anyone in Occupied China have a chorus chanting it (instead of, perhaps, "Deutschland über Alles") is beyond me.