Castille Strategy
Introduction
Playing with Castille is very easy.You are one of the wealthiest countries in Europe with good expansion probabilities.
Beginning
You have 17 provinces, all in western Europe. One of them is a trade centre that gives you some profits in the beginning.
Threats
Your only possible threat is France, but in the beginning you don´t have any boarder with them. They aren't as dangerous as England because their fleet doesn't match yours. Try to ally them, but only when Aragon is conquered, to prevent France to atack Aragon when you do so.
Objectives
As Castille you need to have a good fleet as well as a big army to defend your boarders. I suggest a total of 50k army and a 40 ships fleet
Castille must also pay attention to Portugal, destroying them, before they start colonizing, otherwise, many of your pretended colonies, you'll belong to them. Without Portugal in the map, you have the whole Atlantic, Indic and Pacific for you for around 50 years.
One of your objectives will be very probably "form Spain", but you have to conquer Aragon and have cores there before doing so.
Colonize America, try to conquer the Gold from the Aztecs and the Incas, but pay attention to the revolts there.
Forget Africa completely because the provinces won't give you any money.
Focus on India and take an 10k army to kill any savages that try to take you out of that land. Don't forget Eastern Asia, because the proinces are very valuable and rich and give you a lot of money.
Before you advance into colonization, try to conquer the rich provinces in Nothern Italy which also have Universties that give you some Stability Investment.
Greece would also be a good place to expand to, but watch out for your Badboy (Reputation) that instantly 'kills' your merchants.