A wind turbine tower assembly device comprising a tower interface configured to be attached to a wind turbine tower during construction of the wind turbine tower, a nacelle interface for being attached to a part of a nacelle of a wind turbine and arranged to place the nacelle above a free end of a wind turbine tower which is attached to the tower interface; a lifting structure configured to move the nacelle interface relative to the tower interface to create a space between the free end of the wind turbine tower and the nacelle interface; a unit receiving structure configured to insert a tower unit into the space; and a tower assembly structure configured for joining the tower unit to the free end of the tower.