System for assembling/disassembling windmills, specifically designed to provide a set of devices and means enabling to carry out the necessary operations for the assembly of all the components that make up a windmill, wherein the system comprises: - a main structure capable of providing one or two hoisting points at different heights using the tower itself of the windmill under construction as a holding pillar, constituting the load holding element during the operations of ascent/descent of the elements of a windmill, capable of varying its geometry and avoiding collision with the tower, nacelle and full rotor during the ascent/descent of the structure by means of mechanisms acting the role of small, electrically operated cranes and also capable of transmitting the stresses directly to the tower without the need for being supported in less resistant parts such as the nacelle or the rotor, by arms with supports that generate pressure; - means for adjustment to the different diameters of the tower along its height, incorporated in the body of the main structure, consisting of multiple pressure arms which, through a rigid structure added to each pressure arm, can adjust its position by a hydraulic actuator and generate working pressure through another hydraulic cylinder, wherein the working pressure can be maintained by mechanical means such as springs in the event of failure of the hydraulic system; - an intermediate fixing structure, capable of being fixed to different tower segments by the same fixing mechanisms as described above consisting of the multiple pressure arms, creating a handling point for tower segments below the top flange and above the centre of gravity of the segment and considerably reducing the necessary height for the assembly of tower segment; - lifting means, already mentioned, arranged on the intermediate fixing structure and consisting of 4 small, electrically operated cranes capable of hoisting/lowering the main structure along the windmill tower until reaching the various working positions, as well as to lower the main structure to the removal position; - a structure in the form of an auxiliary beam providing a single hoisting point by joining together two hooks associated with a pulley system and linked to two pivoting arms at both ends of the auxiliary beam with electric and hydraulic mechanisms enabling the translation and rotation of a further hook for the maneuvers to adjust and position the various components; - a positioning structure placed on the ground with elements indicating the correct position for hoisting the tower segments (1, 1') for its correct positioning without the need for rotation when they are handled through the intermediate fixing structure. - a main structure capable of providing one or two hoisting points at different heights using the tower itself of the windmill under construction as a holding pillar, constituting the load holding element during the operations of ascent/descent of the elements of a windmill, capable of varying its geometry and avoiding collision with the tower, nacelle and full rotor during the ascent/descent of the structure by means of mechanisms acting the role of small, electrically operated cranes and also capable of transmitting the stresses directly to the tower without the need for being supported in less resistant parts such as the nacelle or the rotor, by arms with supports that generate pressure; - means for adjustment to the different diameters of the tower along its height, incorporated in the body of the main structure, consisting of multiple pressure arms which, through a rigid structure added to each pressure arm, can adjust its position by a hydraulic actuator and generate working pressure through another hydraulic cylinder, wherein the working pressure can be maintained by mechanical means such as springs in the event of failure of the hydraulic system; - an intermediate fixing structure, capable of being fixed to different tower segments by the same fixing mechanisms as described above consisting of the multiple pressure arms, creating a handling point for tower segments below the top flange and above the centre of gravity of the segment and considerably reducing the necessary height for the assembly of tower segment; - lifting means, already mentioned, arranged on the intermediate fixing structure and consisting of 4 small, electrically operated cranes capable of hoisting/lowering the main structure along the windmill tower until reaching the various working positions, as well as to lower the main structure to the removal position; - a structure in the form of an auxiliary beam providing a single hoisting point by joining together two hooks associated with a pulley system and linked to two pivoting arms at both ends of the auxiliary beam with electric and hydraulic mechanisms enabling the translation and rotation of a further hook for the maneuvers to adjust and position the various components; - a positioning structure placed on the ground with elements indicating the correct position for hoisting the tower segments (1, 1') for its correct positioning without the need for rotation when they are handled through the intermediate fixing structure.