The most recent Europa Universalis IV dev diary takes a good look at Spain and how it impacts gameplay.
This is the first dev diary to focus on a single nation, but they definitely chose a good one. Spain represents a lot of influence and wealth from the time period that Europa is set in.
Spanish influence was quite heavy at this point in history, with many of the key historical events that take place linked to Spain and it’s dominance.
Spain is a popular choice for many players due to it’s pre-eminence and chances for expansion in whatever direction you might want. For a burgeoning empire, it’s strategically brilliant (So long as you can deal with that pesky France).
The sheer wealth that is available to you gives you a lot of freedom to play the game however you might want.
If this hasn’t sated your appetite then head over to read the dev diary in full.