There are total 5 Secret levels in the game. You’ll have to find and collect 5 maps to unlock these levels. These levels will appear as new area when you access the map in your camp.
Creepy Crypt Secret Level
You’ll get it inside a crypt in Creeper Woods. Location of the crypt is shown in picture below. Enter the crypt and interact with the map in front to unlock Creepy Crypt Secret Mission.
Soggy Cave Secret Level
You’ll get it inside a crypt in Soggy Swamp. Enter the crypt and make your way to a small pedestal. As you approach it, the gave will shut close and you’ll have to fight some enemies. After the fight is ovet the gate will open. Interact with the pedestal it will create a path to reach the map. Interact with the map to unlock Soggy Cave Secret Mission.
Arch Haven Secret Level
It can be unlocked on a pirate ship in Pumpkin Pastures. This entrance point has an extremely rare spawn chance. Complete previous 2 secret levels In hard difficulty to increase the spawn chance for the secret entrance to appear. Go through the secret entrance and defeat the enemies and then interact with the map to unlock Arch Haven Secret Mission.
Underhalls Secret Level
You’ll get it in Highblock Halls.Just after the level starts, move a little forward and get down the stairs. Turn around and you’ll see 2 shields hanging on the wall. You will be able to interact with one shield to open a secret entrance. Inside you’ll find a map to unlock Underhalls Secret Mission.
??? Secret Level (Secret Cow Mission)
To unlock the secret cow mission first you have to complete the entire main campaign. After that you’ll have to collect 9 Secret Runes from each main campaign levels. After that go to the church in your camp and interact with the wall at the far end to reveal a secret room. Inside the room you’ll find the map to reveal the secret cow mission.