At the bottom of the stairs in the first tower there is a wooden partition carrying a "Munition" sign, right opposite the door. Break the partition for some ammo.

Same tower, two floors higher, in the radio room: steal the binocular first. After that, destroy the pinboard, and the wall behind it - there is some hidden gold.
By the way, in this game, most items can be destroyed. Have your evil little fun.

In the dormitory that you enter from the yard, there are some bad floor tiles. Break them.

In a room with a broken floor, somebody shoots at you from below. Put the grunts out of their misery, drop down through the hole and break a hole in the bricked-up door opening.

Yet another room with a fireplace. Look for a crooked-hanging unlit candleholder. Nudge it, and a secret room will open.

A couple of doors further away, there is a big room with a few Nazis and a map board. Destroy the picture of Hitler, there is gold hidden behind it.
Just out of principle, always shoot Hitler! ;)

You arrive at a 'U' shaped corridor with grey cutout stone wall arcs. Press the decoration to the left - the righthand one will open and reveal a secret room.