I think that Ramelli may not have been the jolly book wheel-inventing chap that scholars have cast. He may have been something else.
Consider his relationship with the Marquis de Marignano. The Marquis was the butcher of Siena who turned the siege of 1555 into a bloodbath. The Marquis--and probably Ramelli--was also at the Siege of Metz in 1552 where the French repelled Charles V. The Duke of Alba also lent his unique (and brutal) talents to this particular initiative.
Marignano died in 1555 or 1556. Parsons blithely notes that Ramelli went to France following the death of Marignano and entered service with the Duke of Anjou, future King of France... who was four years old at the time. Not likely. Ramelli probably hung around Italy for a few more years. As noted by Pepper and Adams, the Italian wars didn't wind down until 1559 and there was subsequently a great deal of fortification work going on. Paciotto's citadel in Turin, for example, was built between 1564 and 1566. There is some evidence for Ramelli's presence in Turin during these years. A manuscript copy of Cataneo's Rota Perpetue (1562) is held in the University of Turin Library.
The 1560s seem to be some pretty lean years for military engineers. There were the preliminary forrays of the French Wars of Religion with action in Jarnac and Orleans. Real engineering work didn't really start up until 1567 when the Duke of Alba started his campaign in the Netherlands (and began construction of Paciotto's design for the Antwerp citadel). I'm assuming that Ramelli tagged along and then moved on to France in 1572 following Alba's breakdown and military disasters. Following the St. Bartholomew's Day massacre, it seems that Catherine de Medici was looking for some technical professionals who knew their way around siege massacres in anticipation of the attack on La Rochelle. A veteran of Metz, Sienna, Oosterweel, Rheindalen, Heiligerlee, Jemmingen, Jodoigne, and--perhaps--Haarlem would have been beneficial.