It is tremendously easy to avoid war with Castile, you ally Portugal, The Pope and someone like Bohemia or Austria. If you do get into war, it will be in your favour. You can easily turtle until colonization. Otherwise, they are a fine nation for destroying Europe that isn't the obvious first choice of France.
The bottom line is that they have fiddled with their AI in such a way that the AI is suffering v itself. Lucky nations have basically no AE, increased troop counts, and tend to be the biggest = blob. Since they have no real AE, they don't suffer coalitions like the players do = Blob. You have to outblob them, it easiest to do this at the beginning of the game to ensure they never blob.
Castile is not the easiest nation to do this with, but not the worst, and it's my favourite nation to play.