In the very first room of the dungeon, search the south wall to find a hidden switch. Pressing it will open a secret room behind the south wall.
Reward: Lurker Boots, Shuriken
In the northern chamber near the blind statue puzzle, in the cell containing one of the Blue Gems, you'll find a switch on the wall. Press it, and head straight north to find a hidden room containing some supplies.
Reward: Dagger, Tar Bead, Cave Nettle x2
This secret is contained in one of the game's Iron Doors. You'll find it near the stairs leading down to Level 2 - Old Tunnels. Opening it is actually fairly simple - there's two hidden switches on the walls of the room, one of them at the northwest end, the other at the southeast. Press both of them and the Iron Door will open up.