Then, in the year 1541, his wife died, leaving him two boys; and he remained until 1543 without coming to any further resolution about his life. At that time, in the month of September, there appeared in San Marino one Signor Gustamante, a Spaniard, sent by his Imperial Majesty to that Republic on some affairs. Giovan Battista was recognized by him as an excellent architect, and at his instance he entered not long afterwards into the service of the most illustrious Lord Duke Cosimo, as engineer. And thus, having arrived in Florence, his Excellency made use of him for all the fortifications of his dominion, according to the necessities that arose every day; and, among other things, the fortress of the city of Pistoia having been begun many years before, San Marino, by the desire of the Duke, completely finished it, with great credit to himself, although it is no great work. Then, under the direction of the same architect, a very strong bastion was built at Pisa. Wherefore, his method of work pleasing the Duke, his Excellency caused him to construct--where, as has been related, there had been built on the hill of S. Miniato, without Florence, the wall that curves from the Porta S. Niccolo to the Porta S. Miniato--the fortification that encloses a gate by means of two bastions, and guards the Church and Monastery of S. Miniato; making on the summit of that hill a fortress that dominates the whole city and looks on the outer side towards the east and the south, a work that was vastly extolled. The same Giovan Battista made many designs and ground-plans of various fortifications for places in the states of his Excellency, and also various rough models in clay, which are in the possession of the Lord Duke. And since San Marino was a man of fine genius and very studious, he wrote a little book on the methods of fortifications; which work, a beautiful and useful one, is now in the possession of Messer Bernardo Puccini, a gentleman of Florence, who learned many things with regard to the matters of architecture and fortification from San Marino, who was much his friend.