"Fortification was the principal reason for Philip's eagerness to acquire engineers. The new Italian system with its bastions and regular polygonal trace had slowed the pace of sixteenth-century land warfare, keeping the enemy at a distance and eliminating the former superiority of artillery attack. The men who know how to construct the impregnable defences were in great demand throughout Europe. In October 1561 Philip wrote to the duke of Savoy requesting the loan of Francesco Paciotto of Urbino, the working in Turin. He wanted to consult the engineer on fortifications, and urgently: 'the quicker you send him to me the happier I will be.' Paciotto subsequently fortified La Coruña and designed the famous citadel at Antwerp. When Granvelle was viceroy of Naples, Philip asked him for a detailed report on engineers in the kingdom, their experience and their salaries, with a view to bringing them to Spain. Granvelle replied that none of the nine he listed had the necessary experience, though he singled out 'Jacobo, a Fleming' who was 'not very expert in fortification but is very well trained in mathematics and mechanics, has a fine intellect and so we can expect very good service from him.'
News on engineers, indicating competence in fortification, current salary, and purity of blood, was also sent to the king from the embassies. There was a certain amount of intrigue in attempting to lure to Spain or parts of Philip's empire engineers employed in the service of other princes. From the embasy in Venice Philip's empire engineers employed in service of other princes. From the embassy in Venice Philip learned that Orologio of Vicenza, an engineer 'who is reputed to have a better understanding of the art of fortification than anyone else in Italy' was discontent in spite of the high salary given him by Signoria. Word had reached the Spanish embassy that he would be willing to serve Philip instead and this was confirmed in secret discussions with him. The embassy asked the king for quick decision, emphasizing that, apart from his familiarity with Venice, 'this man knows every corner of France'. This soliciting of engineers could cause resentment when it resulted in the loss of a valued military expert. Venice protested to the Spanish ambassador when Giovanni-Paulo Ferrari left to serve Philip in Naples. Philip was sometimes the loser. In 1572 he heard from his envoy in Turin that an Italian engineer who had recently completed fortifications in Flanders had become disgruntled and switched his services to France. There was concern about what this man might do for the enemy and Philip commented that 'it would be desirable to know his name' and inform the duke of Alva, captain-general in the Netherlands."
It's interesting--to me at least--that this mysterious engineer went to France in 1572, the same year that Ramelli appears at the seige of La Rochelle in the service of Henri, Duke of Anjou.
Goodman, David C. (1988) Power and penury: Government, technology, and science in Philip II's Spain.