Francesco di Giorgio Martini's fortress complexes, created at the end of the Quattrocento, continue to look experimental and highly speculative half a millenium 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.