A method of assembling a tower section of a wind turbine tower is provided. The method includes arranging the tower section of the wind turbine tower in a horizontal orientation; transporting a damper unit to a position inside the tower section while the tower section is arranged in the horizontal orientation; and mounting the damper unit to the tower section, wherein the damper unit is configured to damp motions of the wind turbine tower. Further, a horizontal transport system is provided that is supported at least at one position by the tower section and that includes a movable part configured to support a damper unit and to transport the damper unit in a horizontal direction from a position adjacent to an end of the tower section to a position at or adjacent to a mounting position of the damper unit inside the tower section.