The Donjon--"Keep" in English--was a fortified tower, a citadel, that was independent of the walls of a fortress but could still protect the city. According to Viollet-le-Duc, the donjon was a direct consequence of the feudal system. A Lord was dependent on his vassals, but vassals weren't trustworthy. If they betrayed him, he needed a back-up plan. Thus, come the revolution, the lord of the domain could hole up in the donjon (as happened during the Albigensian Crusade when the wife of Simon de Montfort shut herself and her people up in the keep of the Chateau Narbonnais in Toulouse.) According to Viollet-le-Duc, there was no château without a keep just as there was no city without a château.
The 1525 donjon in Toulouse then, though petite compared to its 13th and 14th century models, fit the criteria of a stand-alone defensive tower in which people could take shelter should their enemies approach. In the case of 16th century Toulouse, the enemy was Spanish.
The first image, a old carte postale, shows the towering donjon of Coucy, the tallest in France at 55 meters high, which was built in 1220. The Lords of Coucy defended Paris to the north and were powerful enough that they proudly boasted, "Neither king, nor prince, nor duke nor count am I. I am the Lord of Coucy." The donjon was blown up by the Germans in World War I out of spite.
The second image is a donjon from one hundred years later. Things have gotten square and towers have been added.
The third image is a rather romantic view of the Temple in Paris built by the Knights Templar from the 13th century. A building so grim always becomes a prison in France--Louis XVI and Marie Antoinette were imprisoned there, in fact--and by the time of the Revolution, the landmark had become a symbol of all that was oppressive about the ancien regime. It was demolished in the first part of the 19th century.
The final image is a Viollet-le-Duc drawing of the 1400 donjon de Vez. I am not sure if the cone roofs were original or another fanciful excursion by Viollet-le-Duc (the donjon today does not have them) but it does resemble the donjon in Toulouse.