pen attesting the onerous duty imposed upon him by Federigo, as his military engineer. In July 1478 he was attached to the allied army, which the Duke commanded; and, in his autograph MS. speaks of having a hundred and thirty-six "edifices" on hand at once by his order. Among these, doubtless, there were many strongholds in the duchy; and he has left descriptive plans of Cagli, Sasso Feretro, Tavoletta, and Serra di S. Abondio. From various authorities cited by Promis, we may add, as probably of his construction, Castel Durante, S. Angelo in Vado, Orciano, S. Costanzo, S. Agata, Pietragutola, Montecirignone, S. Ippolito, Montalto, La Pergola, Cantiano, Fossombrone, Sassocorbaro, Mercatello, Costaccioro, Mondavio, and Mondolfo, besides numerous churches which he certainly planned for Federigo. The fortresses of Urbino have been estimated at nearly three hundred, a number which must seem at once superfluous and incredible, but for the entire change which the arts of war and defence were then undergoing, consequent on a general introduction of artillery.[*159] Federigo, perceiving the importance of strengthening his castles and citadels against "The cannon-ball, opening with murderous crash The way to blast and ruin," not only kept in active employment the most able engineer whom Italy then possessed, but, according to that artist's testimony, by his own experience and judicious suggestions, greatly facilitated the tasks which he imposed upon Francesco di Giorgio. [Footnote *156: There is a predella picture by him at S. Domenico, in Siena, and another in the Uffizi Gallery. He was the pupil of Vecchietta.] [Footnote 157: See vol. I., pp. 147-50, 161-3; _Lettere Sanesi_, III., p. 79; _Carteggio d'Artisti_, _passim_, I., pp. 255-316.] [Footnote *158: Cf. also BORGHESE & BANCHI, _Nuovi Documenti per la Storia dell'Arte Senese_ (Siena, 1898).] [Footnote *159: On the fortresses of the Marche generally, see GASPARI, _Fortezze Marchigiane e Umbre_, in _Arch. St. per le Marche e per l'Umbria_, vol. III., p. 80 _et seq._] Nor was it his professional services alone which the Sienese artist placed at his patron's disposal. The documents published by Gaye and Promis show him accredited on various occasions as the Duke's envoy to the government of his native city; and his _Liber de Architectura_ is dedicated to Federigo, at whose request, probably, it was composed. Vasari adds that he portrayed him both in painting and on a medal; and, in return perhaps for these diversified labours, that prince thus interceded for his admission into the magistracy of Siena. "Mighty and potent Lords and beloved Brethren;