Teester
Thanks.  Merlin Castle was actually mapped in OpenStreetMap, but the standard map layer does not display it as it was tagged.  You could see it marked on HistOSM (http://histosm.org/#16/-8.99607/53.28062/0/)

I've added details to it in changeset #51989496 and it is now displayed.

Please note that the Google Streetview images are copyright of Google and, as Google have not given explicit permission to use them, they cannot be used as a source to map things in OpenStreetMap