The 17th-century treatises on fortification architecture discussed in this book reveal preferred concepts and manners of fortification, which allows for comparison with other historic fortifications and a historical assessment of their military capabilities. Particularly, these treatises include the contemporary theoretical debates as well as the spectrum of the actual activities, which reflects practical facets like building trade, innovations in weaponry, scientific methods, and technologies of attack. The analysis documents that around 1600 there was a paradigm shift to accommodate new procedures of siege, marking a caesura in international fortification architecture. The study embraces about 200 treatises to develop the theory of fortification in which the aspect of the military mainly constitutes the shape of the "Architectura Militaris.".