Mein Code sieht so aus:

Klasse a.hpp

#ifndef A
#define A

class b;

class a : public x {
 b* nn;
//bunch of stuff
};  //<--- hier wird der Fehler geworfen (laut compiler)

#endif

a.cpp

#include "a.hpp"
#include "b.hpp"
//stuff

Klasse b.hpp

#ifndef B
#define B

class a;

class b {
 a* nn;
//stuff
}

#endif

b.cpp

#include "b.hpp"
#include "a.hpp"
//stuff

Wo habe ich den Murks gebaut?^^

#ifndef STATIONABLE
#define STATIONABLE

class Moveable;

#include "Model_Placeable.hpp"
#include "../util/Enums.hpp"
//#include "../unit/Model_Moveable.hpp"

class Stationable : public Placeable {
	Moveable* occupier;
	Type kind;

public:
	Stationable(Point,Point,Type);
	Moveable* get_occupier();
	void set_occupier(Moveable*);
	Type get_type();
	virtual ~Stationable() = 0;

};

#endif

Stationable.cpp

#include "../unit/Model_Moveable.hpp"
#include "Model_Stationable.hpp"

Moveable.hpp

#ifndef MOVEABLE
#define MOVEABLE

class Stationable;

//#include "../area/Model_Stationable.hpp"

class Moveable {
	Stationable* stationed;

public:
	Moveable(Stationable*);
	Stationable* get_stationed();
	virtual void set_stationed(Stationable*) = 0;
	virtual bool can_stationed(Stationable*) = 0;
	virtual ~Moveable() = 0;
};

#endif

Moveable.cpp

#include "../area/Model_Stationable.hpp"
#include "Model_Moveable.hpp"

Compiler:

g++ -c ./area/Model_Stationable.cpp
In file included from ./area/Model_Stationable.cpp:2:
./area/Model_Stationable.hpp:25: error: multiple types in one declaration