A security door comprises a frame structure, having a lateral surface in which a plurality of doorways are constructed. A first doorway has a first width along an inner circumference of the frame structure, and lateral surface portions delimiting the first doorway in a first direction and a second direction, opposing the first direction, along the inner circumference of the frame structure have a second width. A lock element is received in the frame structure and comprises a first segment, movable along the frame structure inner circumference relative to the frame structure, and a second segment, movable independently of the first segment along the inner circumference relative to the frame structure. The first segment is dimensioned so that, in a position in which it closes the first doorway, it also closes a second doorway adjacent to the first doorway in the first direction along the frame structure inner circumference.