How should the castle be positioned to allow access to the shipyard?

Perhaps this has already been discussed, but I couldn't find it. How do you properly position a Castle in the editor so that it's possible to build a Shipyard? I've already placed it right next to the water; it's physically impossible to place it any closer, but I still can't build it. If I try to build it manually, it says that the Shipyard is abandoned, meaning it still doesn't work. How the heck do you position this Castle?