Annex them. No, really, as Castile you can pretty easily take down Portugal from the get-go, and then take your time picking apart France and England, since they take a long time to start colonizing anyways. And of course, France if France, and can easily stomp all over Castile before they so much as take Exploration, or at least before they can do anything special with it. England is harder, unless you can win the HYW, but still very doable.