C++ forward declaration error

I have an error that goes like this

    In file included from Level.hpp:12,
                 from main.cpp:4:
Corridor.hpp: In method `void Game::Corridor::update()':
Corridor.hpp:41: invalid use of undefined type `class Game::Level'
Corridor.hpp:13: forward declaration of `class Game::Level'
Corridor.hpp:42: invalid use of undefined type `class Game::Level'
Corridor.hpp:13: forward declaration of `class Game::Level'
Corridor.hpp:43: invalid use of undefined type `class Game::Level'
Corridor.hpp:13: forward declaration of `class Game::Level'
Corridor.hpp:44: invalid use of undefined type `class Game::Level'
Corridor.hpp:13: forward declaration of `class Game::Level'

Corridor and Level are ...

  // Corridor.hpp

#ifndef GAME_CORRIDOR_HPP
#define GAME_CORRIDOR_HPP
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#include <Moot/Math.hpp>

//#include <Level.hpp>
#include <GameWindow.hpp>

namespace Game
{
    class Level; // <-- LINE 13

    class Corridor
    {
        static const unsigned int defaultLevelDepth = 800;

        Moot::Math::Vector3D wp1, wp2, wp3, wp4;
        Moot::Math::Vector2D sp1, sp2, sp3, sp4;

        Level * p_level;

    public:

        Corridor(Moot::Math::Vector3D setFirstPoint, Moot::Math::Vector3D setSecondPoint) 
        {
            wp1 = setFirstPoint;
            wp2 = setSecondPoint;

            wp3 = setFirstPoint;
            wp3.z += defaultLevelDepth;

            wp4 = setSecondPoint;
            wp4.z += defaultLevelDepth;
        }


        void update() {

            sp1 = p_level->getLevelCamera().convert3DVectorWithScreenAlgorithm(wp1); // <- LINE 41 etc.
            sp2 = p_level->getLevelCamera().convert3DVectorWithScreenAlgorithm(wp2);
            sp3 = p_level->getLevelCamera().convert3DVectorWithScreenAlgorithm(wp3);
            sp4 = p_level->getLevelCamera().convert3DVectorWithScreenAlgorithm(wp4);


            //p_level->getLevelCamera();
        }

        void draw()//const
        {
            Moot::Color tempColor;

            windowInstance().graphics().drawQuad( sp1.x, sp1.y, tempColor,
                                                                                sp2.x,sp2.y, tempColor,
                                                                                sp3.x, sp3.y, tempColor,
                                                                                sp4.x,sp4.y, tempColor, 1);
        }


        void setLevel(Level* setLevel) {
            p_level = setLevel;
        }

    };
}

#endif

and

// Level.hpp

#ifndef GAME_LEVEL_HPP
#define GAME_LEVEL_HPP

#include <Moot/Forward.hpp>
#include <Moot/Window.hpp>
#include <Moot/Math.hpp>

#include <GameWindow.hpp>
#include <Camera.hpp>
#include <Corridor.hpp>
#include <Player.hpp>

#include <vector>


namespace Game
{
    class Level
    {

        typedef Corridor* p_corridor;
        typedef std::vector<p_corridor> CorridorList;
        typedef CorridorList::reverse_iterator ReverseCorridorItter;

        CorridorList m_map;
        Camera       m_camera;
        Player         m_player;


    public:

        Level()
        {
            m_player.setLevel(this);

            // Lots of vertices being defined into m_map.


            // Loop through and set camera
            ReverseCorridorItter rit;

            for(rit = m_map.rbegin(); rit != m_map.rend(); rit++)
                (*rit)->setLevel(this);


        }


        ~Level()
        {
            ReverseCorridorItter rit;

            for(rit = m_map.rbegin(); rit != m_map.rend(); rit++) 
                delete (*rit);

            m_map.clear();
        }


        void update() 
        {
            // Temp delete when input and player are implimented.
            if(pad[0].buttons & PAD_UP)
                m_camera.updateTargetOffsets(0, -2);

            if(pad[0].buttons & PAD_DOWN)
                m_camera.updateTargetOffsets(0, 2);

            if(pad[0].buttons & PAD_LEFT)
                m_camera.updateTargetOffsets(-2, 0);

            if(pad[0].buttons & PAD_RIGHT)
                m_camera.updateTargetOffsets(2, 0);

            m_player.update();


            ReverseCorridorItter rit;

            for (rit = m_map.rbegin(); rit != m_map.rend(); rit++)
                (*rit)->update();
        }


        void draw() // const // EH!!! wtf ReverseIter isn't a member
        {
            m_player.draw();


            ReverseCorridorItter rit;

            for (rit = m_map.rbegin(); rit != m_map.rend(); rit++)
                (*rit)->draw();

        }


        Camera& getLevelCamera() {
            return m_camera;
        }

    };
}

#endif

The pointer is being set as far as I can tell, but when I try to access a function from Level, BOOM!

Thanks.

PS: The compiler is gcc 2.95.2 if that makes a difference.

EDIT

Updated with complete code.

You are #include-ing Level's complete declaration:

#include <Level.hpp>

...and then you try to forward-declare Level:

namespace Game
{
    class Level;

Don't do this. Choose one or the other. (edit) Or at least put the forward-declaration before the #include-ion of the complete declaration. If all you're doing in game_corridor.hpp is setting pointers to a Level, then a forward declare should do fine. If however you need to call functions on Level from within the HPP file, then you'll need to #include the complete declaration.

EDIT2:

Based on your clarifying edit to your OP, you must #include the complete declaration of Level, and not try to use a forward declaration.

If you forward-declare Game::Level then don't #include it. In a not-so-related note, use #include "header.hpp", not #include <header.hpp>.

Edit as per your updates: Bring the definition of Game::Corridor::update() outside the header and into an implementation file. This way the compile need not know anything about Game::Level apart from the fact that it exists and it's a type.

The problem is that Corridor doesn't know what a Level is, because it can't really #include Level.hpp, because Level.hpp is what #included Corridor.hpp.

The underlying problem is that you're trying to #include a source file. The really underlying problem is that you're using #include when you haven't separated your code into source files and header files. Here's how to split it up. (I'm assuming you're familiar with compiling source files into object files, then linking them into executables.)

Corridor.hpp:

#ifndef GAME_CORRIDOR_HPP
#define GAME_CORRIDOR_HPP

#include <Moot/Math.hpp>

#include <Level.hpp>

namespace Game
{
  class Level;

  class Corridor
  {
    static const unsigned int defaultLevelDepth = 800;

    Moot::Math::Vector3D wp1, wp2, wp3, wp4;
    Moot::Math::Vector2D sp1, sp2, sp3, sp4;

    Level * p_level;

  public:

    Corridor(Moot::Math::Vector3D setFirstPoint, Moot::Math::Vector3D setSecondPoint);

    void update();
    void draw();

    void setLevel(Level* setLevel);
  };
}

#endif

Corridor.cpp:

#include "Corridor.hpp"

namespace Game
{
  Corridor::Corridor(Moot::Math::Vector3D setFirstPoint, Moot::Math::Vector3D setSecondPoint) 
  {
    wp1 = setFirstPoint;
    wp2 = setSecondPoint;
    wp3 = setFirstPoint;
    wp3.z += defaultLevelDepth;
    wp4 = setSecondPoint;
    wp4.z += defaultLevelDepth;
  }

  void Corridor::update() 
  {  
    sp1 = p_level->getLevelCamera().convert3DVectorWithScreenAlgorithm(wp1);
    sp2 = p_level->getLevelCamera().convert3DVectorWithScreenAlgorithm(wp2);
    sp3 = p_level->getLevelCamera().convert3DVectorWithScreenAlgorithm(wp3);
    sp4 = p_level->getLevelCamera().convert3DVectorWithScreenAlgorithm(wp4);
  }

  // and so on

}