This article presents the manuscript document, Letters, instructions and decrees by the engineer Bautista Antoneli or Antonelli, the most important military architect of the Spanish Caribbean fortification system. Handwritten by Antoneli, and little known in the scholarship on sixteenth- and seventeenth-century military defense, the manuscript contains reports, royal decrees, and illustrations relevant to Spain’s defense plan during the last quarter of the sixteenth century. This article describes the document, the context of its production, and its structure. Special attention is given to two main themes or key sections of the book: the journey to the Straits of Magellan and its map, which was based on second-hand information despite the engineer’s claims that he explored the straits himself; and a report and map of a bi-oceanic project in the Chagres River, in the Central American isthmus, an idea three centuries ahead of the development of the Panama Canal. As such, the map may be the first graphic document of the future interoceanic canal.