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

Got it. The X-shaped structures themselves weren't active, but there was a slider below – 2/3 of that menu was off-screen, and that's where all the values are entered. Thanks for the help! :)
P.S. I hope you need to assign them to the nearest water point to the slope, and not to the farthest point of the "slope." As far as I remember, yes, the ships are stationed there.