I think it is possible. You have the dominant military and naval force. That said, it will come down to how much stability hit/infamy hit you are willing to take. Aragon still exists, Portugal still exists, and all the African countries still exist, so the only way you could weaken Castille at this point would be to either annex some land yourself (4bb per province) or force them to release Navarra and Galacia, which wouldn't really weaken them at all. If your infamy cap is 30, the worst you could do to them is take 6-7 provinces, but then you will have maxed out your infamy and would be spending the next 30-50 years fighting rebellions, having horrible diplomatic hits, and not be able to trade for crud. Do you have any real casus belli against them? Maybe guarantee Aragon, Portugal, Morocco, Tunis, and Algiers and wait for Castille to declare? Then attack. At least then you won't have the stability hit and you can let one of the other countries take some provinces which would let you slow down Castille while not killing yourself with BB.
