I've read the fascinating book(s) by Braudel on the Mediterranean in the time of Philip II. of Spain and he often writes about the king "raising" or "recruiting" troops in Spain, Italy or Germany for his various military campaigns. It also regularly mentions that these troops are again disbanded in Winter.
So how exactly did that work? Was it a semi-professional army? Was it just mercenaries? Did always the same or different people get employed by the Spanish army? Was it different bands of mercenaries?
I'm trying to e.g. compare it to the ancient Roman military or a modern army. Both (the Romans after a certain point in time) had professional armies, i.e. all or most soldiers only professionally worked in that capacity, were employed for a certain time (e.g. 15 or 20 years), wore uniforms and mostly lived in or near military camps. Was it the same or different for Spain in the 16th century? How exactly?
/u/TooDriven the keyword you are looking for is the "fiscal-military state", or the rise of.
Here's a starting point: link. There was a discussion on this in an old AMA.
At the time of Philip II of Spain, the funding model tended to be:
Discontinuous. There was limited continuous transferrable fiscal tools available, so there were gaps in funding that enterprisers at all levels had to compensate against. For example, the Spanish army suffered through many mutinies because of this.
Sovereigns only provided partial funding, or promises of reimbursement, when an army has to be raised. A good example is the Army of Flanders, raised by the Duke of Alba in 1566-1567, that relied on the Duke himself for most of the advance payments and maintenance of the army. Infamously, he enacted a "tenth penny" tax on the Low Countries, which was widely unpopular.
Hopefully, this is a useful starting point.