His treatise catered explicitly to the needs of architects, builders, and craftsmen. The treatise is composed of eight books, the sixth of which was lost for some centuries and the eighth of which was not published until relatively recently. The eighth book is not always considered to be part of the treatise. The first five books cover Serlio's works on geometry, perspective, Roman antiquity, the orders and church design. The sixth illustrates domestic designs ranging from peasant huts to royal palaces, providing a unique record of Renaissance house types, including up-to-date fortresses for tyrants and mercenaries as well as Serlio's unbuilt design for the Louvre. The seventh book illustrates a range of common design problems ignored by past theorists, including how to remodel, or 'restore', Gothic façades following antique principles of symmetry and proportion. The eighth book, called "Castrametation of the Romans", reconstructs a Roman encampment after the description by Polybius, followed by a military city and monumental bridge supposedly built by the Emperor Trajan. With its forum, consul's palace and baths, the book is part-fantasy and part-archaeology, quite unlike Serlio's other more practical works. In the introduction to Book IV, Serlio credits his recently deceased mentor for much of its content: "As for all the pleasant things which you will find in this book, you should give the credit not to me but to my teacher, Baldassare Peruzzi from Siena..." The extent of Peruzzi's contribution to the treatise is unknown. "Peruzzi had been the guiding spirit in the detailed study of the remains of antiquity, and he had left his drawings to Serlio. Vasari and Cellini would give most of the credit for the book to Peruzzi, but more recent writers defend Serlio's part in the study and his good faith in completing the work of his companion." Plan and publication By 1537, when the earliest of his books was published, Serlio had been working on the treatise for at least a decade and had already organized it as a work in seven books. Although Serlio completed all seven projected books, only the first five books were published during his lifetime. The sixth remained in manuscript until the 20th century. He composed two additional books, which can be thought of as appendices: the Extraordinary Book of Doors, the last book he saw through the press; and On Polybius' Castrametation, a discussion of ancient Roman military camp design, whose state