At the time of the construction of these and the other lofty towers it was still thought that the raising of batteries as much as possible would increase their effect. In practice the plunging fire from platforms at the height of some eighty feet above the level of the parapets of the town wall can hardly have been capable of producing any great effect, more especially if the besieging force succeeded in establishing itself on the crest of the counterscarp of the ditches, since from that point the swell of the bastions masked the towers. But there was another use for these lofty towers. The fact is that the Nuremberg engineers, at the time that they were built, had not yet adopted a complete system of flank-works, and not having as yet applied with all its consequences the axiom that _that which defends should itself be defended_, they wanted to see and command their external defences from within the body of the place, as, a century before, the baron could see from the top of his donjon whatever was going on round the walls of his castle, and send up his support to any point of attack. The great round towers of Nuremberg are more properly, in fact, detached keeps than portions of a combined system, rather observatories than effective defences.[30] They were perhaps the last of their kind. Tradition has quite incorrectly ascribed them to Albert Durer. Not only were they built thirty years after his death, but they are in principle entirely opposed to the views expounded in his book on the "Fortification of Towns." This book, which appeared in 1527, broke completely with the old mediæval art of fortification (the theory of which may be said roughly to have consisted in an extensive use of towers), and recommended the construction of such bastions as the Köcherts-zwinger, or that in the neighbourhood of the Laufer Thor (1527) which form the starting-point of modern fortification.