Oh well, I'll just keep pally with France and punch the **** out of Austria and co if they mess
Do not. Always break big countries. Always!!!!
Austria is not your enemy if you are Castile. France is, and going to backstab you the moment Austria is gone with the help of the Ottomans some times.
You need French territories to dominate the Genoan node, which is end node and the most profitable to grab the maximum amount from your colonies. With Aragon & Naples you get a great foothold to completely control 100% of it.
As Castile you should focus on PU Aragon & Naples, try to remove from the map completely Portugal (no vassal nothing, eradication as if you integrate them, Brazil would form if has colonies).
Push after that Morocco to break apart and feed all it's territories except those in your home trade node to an North African vassal which is needed. (you save admin and diplo points here).
Also you need to defend this vassal against rebels for 100 years or so, but is fine. You will annex this vassal when you move to Central Africa later on, so feed it everything.
By the time you finish with N Africa your vassal should expand from the West of Morocco, all way to Tripoli and the name of your country should be Spain owning the Italian peninsula.
That is why you need the PU with Aragon & Naples, and feed Naples the Italian peninsula when the Shadow Kingdom appears. (with the help of France initially).
When you mop 3/4 of Italy and feed it to Naples, except the HRE lands, keeping Rome as Papal state (preferably vassal) is the time to start chewing France.
You do not have to take everything. Just break him appart taking couple of territories every few years. Just to avoid coalition wars. (not that would be any)
EU4 is not Total War. Is better to annex vassals 50 years later, than gobble land fighting coalition wars for the next 100 years.
Also as Spain, if you want to take Rome you must become Protestant, or are not any big Catholics in vicinity after Reformation & Protestantism pop.
If you go for massive colonial expansion, you need to be Catholic, as only England will ignore you (if turned to Anglican), and any state from HRE that tries to colonize you get claim to completely remove it's colonies.
Colonial game as Spain/Castile. You need to block everything from Caribbean, Mexico and Chesapeake/Louisiana and Chile. After that expand in Africa.
(usually do 1 Africa, 2/3 in Americas even if you go over the limit) to build colonial nations fast.
Territories in Africa make them companies.
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Btw since you were talking about PU the other day, need to upload the image from the laptop. As a tiny Portugal PU France big blue blob (France/Castile/Aragon/Burgundy/Savoy) and won the war of succession from Brittany
However it was impossible to hold that humongous vassal, and after 60y got the event that broke PU. Need to continue that game, as I have the claim to force PU.
It would be interesting to see how much time it would take to annex the big blue blob since it is 1702. I bet 119 years by end of the game, won't be enough.