Q: Secret #1: Dining room
A: In the fireplace of the dining room, there is a switch. Pullit, the whole fire (!) slides back and an opening in the ground appears. Save... jump down, and you're in a small dungeon with an red-haired, one-eyed zombie in it. She may be quiet or alerted, depending on how much noise she heard so far. There are two fire-crystals/arrows to be found here, and she's got a purse. (If you ignore the warning and press the button, you get trouble). Use the crank and you open the exit to the cellar.
You may notice a silver nugget behind the grate and maybe even the button there. Since the button does not work (in the unpatched version), you need another way there... see below.

Q: Secret #2: Chappel
A: In the chappel, there is a small hammer-symbol on the wall left above the large hammer. Shoot it with an arrow, and the large hammer will slide, revealing an opening in the ground. Down there, there is a haunt and a small maze of tunnels. A plank on the wall can be hacked away with your sword, and behind that you get to the silver nugget seen in secret #1 and its button... see? The button doesn't work.

Q: Secret #3:
A: In the bedroom on the first floor, south of the chappel, turn the torch halter a bit, and you'll open a small secret compartment with loot in it.

Q: not-so-secret
A: In the library, there is a "special" bookshelf and a "special" book nearby. A bit of loot... for some reason, you cannot close the "secret" door. :(

Q: Secret #4:
A: Shoot the second target you encounter in the bull's eye - yes, the one above that door where you run right into this nasty arrangement of camera and turret... A hit opens a compartment with a gas arrow inside.