Offering diverse gameplay mechanics, Castile has long been regarded as a good way to learn various aspects of EU4. With the Domination DLC enabled, Castile makes for more of a challenge by adding early-game internal strife. http://pbs.twimg.com/media/Fw6GYAoaAAAZmeA.jpg
There’s also that pesky infertile heir with the abysmal stats, who until recently was a solid 0/0/0. But regardless, Castile is still a powerful nation, just as it was historically. http://pbs.twimg.com/media/Fw6GdVSagAEQX9r.jpg