There are 27 total secrets. I do not know the exact location of all of them.
number tells you how many secrets in each area
2) Benjamin cellar 3) Castle 1) Museum 2) Zelda forest 2) Orcus castle 2) Golden castle going to St.Moriaz 1) Frozen lake before St.Moriaz gates 1) Stone blocked cave after st.Moriaz 1) Dwarven/Snowhite brewery 1) Treasure of the Fire Demon 1) Basement of the Spectre Tower on the island west of boat harbor 1) Santa House 2) Church near Castle Town 1) House of Error 1) Save the environment tree 1) Yellow ring temple, west of the light bridge 1) I.C.E. Factory 1) I.C.E. Factory armory from Ice Peak Caves 2) Icepeak Castle
Benjamin Cellar- In the room with the picture of benjamin there is a secret passageway to the left of the table that leads to gold
In three way intersection room there is a secret room in between the two statues.
Castle- Enter the Castle from behind and go to the two doors that are locked. They cannot be unlocked untill you pick up the key in Elven town. The key is located inside a box just before entering the passageway to the rings.
Edit- I do not know the location of the third secret at this time (It's at the top left of the castle's interior, walk through the wall and steal the crown).
Bright Warriors Museum in Castle Town- To get in the museum, you have to pay everyone in line (4 people) 20 golz each to skip ahead, and buy a ticket for an additional 20 golz. Once inside, head left, go to the wall with a curtain and head up towards the curtain, and you should pass through. Follow the corridors until you get to a hallway with four doors. One door will have a red carpet in front. Enter that door, a conversation will occur, then when you leave, 500 golz will be waiting for you to pick up.
Church (With nuns with battle tips, north of town)- Just before entering the church, go right, between the statues. Then wind your way down, finding hidden stairs behind the big trees. At the bottom is a treasure chest. Then go back up and do the same on the left, and if I remember correctly, you get gold and a total of two secrets.
Zelda Forest- Behind a tree in the eastern area of the forest before the area with the orcs there is a hidden stairway. For the second secret, as mentioned in the comments, find the arrow drawn with dark water in the north section. Go straight down from the tip into the forest.
Edit- I don't know if this is considered a secret, but if you put Mike as the leader and talk to the fairy who wanders the area, she will teach him 'Healing Song'.
Orcus Castle- After beating the Orcus return to the princess's cell where you will find gold. For the second secret, as you leave the area with the red button (long hallway, going right) just keep going straight at the wall, it is a hidden corridor to a secret area with a treasure chest containing a Masterwork sword.
Forest path to Golden Castle - At one point while travelling through the forest there will be a bridge across some water, and to the right a little ways, right after the bridge, there is a box. When you go to the box you will see a chest to the right around some walls of rock. Ignore the chest for now, this is just for the purposes of location. Now head up a little ways. The main path is to the right, but if you continue up you will see a route barely hidden by trees. Go through there and you've found the secret. Continue up until you get to some ruins with a locked door requiring a golden key, and an open door (this is the golden castle, see below for how to get in.) Inside the open door is some gold, some wine in chests, a clue saying "the gold is in the middle" and a fairy selling more advanced powers.
Golden Castle- Before entering the golden castle, push the action button on the middle pedestal/rock and you will find a golden key. Use it to unlock the castle.
Whilst inside the castle go up the stairs and to the left. There should be a button that when pressed should reveal the secret room
To St. Moriaz- A little ways after the Golden Castle secret, you will get to a snowy area. There will be an island with 2 bridges and 4 columns (1 broken). Note an area to the left of the island with some gold and a chest (contains an ice sling). Continue on the path and when you get to the left-right corridor, note the chest in the top left, and 2 bushes separated by a single space along the bottom. Go between the bushes and continue towards the wall to get to the secret.
Treasure of the Fire Demon- Simply return to the passage way that was previously blocked by the wizard and enter the room filled with treasure
Yellow Ring Temple- Collect all four spagetti balls before proceeding to the light bridge. Insert 3 spagetti balls into pedestals and head north untill you reach the part of the bridge that lets you head west.
If you find any secrets that I neglected to put on here......please put them on. SML