We stayed in Sago Cay room 5441 for our honeymoon in 2000. It's a dormer room with a king bed. High volume ceilings, but a small balcony. And if you're lying in bed, you can't see the castle. But it was great sitting out there on July 4th and watching the fireworks.
I wouldn't request a specific room #, though. Just book a lagoon view room and request a castle view. I've heard that some of the lagoon view rooms in Boca Chica are obstructed by trees. But I've never stayed in that building so I can't speak from experience.