Text in English & German. Francesco di Giorgio Martinis fortress complexes, created at the end of the Quattrocento, continue to look experimental and highly speculative half a millennium later by their semiotic character. They represent an extreme of European architectural history, occupying a position where architecture and sculpture cannot be sharply distinguished any longer. The alien-looking creations represented in this book have their origins in a particular historic situation: the emergence of firearms in the 14th century and their spread in the 15th century had shifted the balance of warfare in favour of the attacking side, against which the defensive structure had not yet found a remedy. Enter Francesco di Giorgio Martini (1439 to 1502) at this point, a native of Siena and one of the Quattrocentos highly versatile artists. He worked mainly in Federico da Montefeltros Urbino, and left behind a body of work that included painting -- the three famous prospects of ideal cities in Berlin, Baltimore and Urbino are attributed to him -- sculpture -- primarily his imposing reliefs -- and architecture -- here he was definitely the outstanding figure between Alberti and Bramante. His achievements as an engineer are equally impressive, and his elaborate designs for machines strongly influenced those of Leonardo da Vinci. He was a true Renaissance uomo universale, though, despite of his voluminous and influential theoretical work, less in the sense of a humanist homme de lettres than as an all-round artist. Francescos sacred and secular structures are classicist and austere in nature, yet his fortress structures look as if, moving beyond all functional concerns, he is exploiting the newness of the task, the lack of any tried and tested technical solutions and the removal of all typological boundaries to give his architectonic fantasies free rein, resulting in an apotheosis of the new, the unfamiliar and the alien. This book is an attempt to understand the strangely grandiose semiotic character of these structures. In doing so, it poses the question of what strategies can be used when seeking a shape for buildings for which there is no precedent.