These can be found on the North/West of walls and shows as a Red Button you can click on. These are hidden all around in many areas of the game.
One of the first ones you can find is in the "Tutorial"/Black Fortress in the beginning. When you are being sent up to talk with Redbeard there's a hidden door in the dead end just past the ruined doors.
Press the RED button on the West wall and door will open.