What is ``The type signature for ... lacks an accompanying binding``?

I am new in Haskell and I had started a little project.
When I want to compile it, I always recieve the error:

[1 of 1] Compiling Main             ( src/main.hs, src/main.o )

src/main.hs:51:9:
The type signature for ‘theDungeon’ lacks an accompanying binding

Now I don't know what this error means or how I can fix it.

Here is the source-code:

import Prelude
import qualified UI.HSCurses.Curses


{-- DEFINE THE TYPES --}
data Tile = Tile  -- the tiles of the map
    {char :: Char
    ,isBlocking :: Bool
    ,position :: (Int,Int)}

data Dungeon = Dungeon [Tile]  -- the dungeon

data Player = Player  -- the player
    {namePlayer :: [Char]
    ,charPlayer :: Char
    ,positionPlayer :: (Int, Int)
    ,lvlPlayer :: Int
    ,epPlayer :: Int
    ,hpPlayer :: (Int, Int)
    ,mpPlayer :: (Int, Int)
    ,damagePlayer :: Int  -- how many damage it does
    ,defensePlayer :: Int  -- how many defense it has
    ,inventoryPLayer :: [Item]
    ,isAlivePlayer :: Bool}  -- it's true if the player isn't dead

data Monster = Monster  -- the monster
    {nameMonster :: [Char]
    ,charMonster :: Char
    ,positionMonster :: (Int, Int)
    ,hpMonster :: Int
    ,giveEpMonster :: Int  -- how much ep the player becomes if it kills the monster
    ,damageMonster :: Int
    ,defenseMonster :: Int
    ,isAliveMonster :: Bool}

data Type = Potion | BigWeapon | SmallWeapon | Scroll | Armor | Amulet   

data Item = Item
    {nameItem :: [Char]
    ,itemType :: Type
    ,positionItem :: (Int, Int)
    ,equipableItem :: Bool}

{-- THE MAIN FUNCTION --}
main :: IO ()
main = do
    let theDungeon :: Dungeon 
    theDungeon <- mkDungeon 0 0 []
    writeFile "./test.log" $ show theDungeon


{-- DEFINE THE SIZE OF THE MAP --}
screenX = 80
screenY = 24

mapX = screenX
mapY = screenY - 4

{-- THE FUNCTIONS FOR THE DUNGEON --}
mkDungeon :: Int -> Int -> Dungeon -> Dungeon -- function for generating the dungeon
mkDungeon x y dungeon =
    if x > mapX  -- looks if the line isn't too long
        then mkDungeon 0 (y + 1)  -- it's too long, so make the next line
        else if y == 0  -- if it at the top
            then do 
                dungeon : Tile '#' True (x,y)
                mkDungeon (x + 1) y  --  make the line full with walls
            else if y > 0 && y < mapY  -- looks if the line is in the middle 
                then if x == 0 || x == mapX  -- is it at the right or at the left
                    then do 
                        dungeon : Tile '#' True (x,y)
                        mkDungeon (x + 1) y  -- make the tile to a wall
                    else do 
                        dungeon : Tile '.' False (x,y)
                        mkDungeon (x + 1) y  -- make it to a floor
                else if y == mapX  -- looks if it is at the bottom
                    then do 
                        dungeon : Tile '#' True (x,y)
                        mkDungeon (x + 1) y 
                    else retu $ dungeon :: Dungeon -- if none of it, the map is finished, retu dungeon

If you (or better 'when') find issues, tell me :D.