Interesting discussion! Thanks to everyone for taking the time to think about this. I didn't want to post an essay in the first place, but I'll now fill in a few more details to answer some of the above questioning parts.
My original idea was NOT to include the castle, and to frame up to the horizon, leaving the buoy in the frame and just a little off centre. Working with a 10 stop ND filter and a polariser, both screw-in, I could not a) see through the lens with them on, b) attach and position an ND grad in addition, to control the sky (I did have one, and considered using it). Also, the sky was rather uninteresting. Plus, since I had to frame, meter, then put the filters on, I decided to *just* get the castle in to give options for cropping it out later, though I also thought radically breaking the 'rule' of thirds might making it interesting or dynamic. In other words, I was giving myself options by using just a little bit of the frame area to capture the castle, whilst also making sure that the buoy didn't end up too near the top of the frame if the camera (perched on slimy weed stuff....) moved at all as I was putting the filters on, post framing and metering.....
I did a crop without the castle and liked it but decided to upload this one as I like it too (indecisive!). I didn't ever see this as a picture of Piel castle, more as a set of shapes which might make a dramatic image; if anything, the pier is the subject (to me!) (perhaps).
James: yes, I'd have liked the buoy lower, but that was as low as I could get it with the tripod at full height :-\ Your crops are pretty much the image I originally intended, and I would have named it something including 'buoy' to point out the subject
Clicknimagine: I also agree that the main subject in the version I uploaded is inevitably the castle, and hence I agree with everything you say.
Newmanp: after this discussion, I agree with you, as per my original vision of the frame prior to making the shot!
I think the rule of thirds rule is more a guide
I still like the dynamic of the thin horizon, though it's a little *too* thin in my original, I certainly won't argue with that. I'd say it comes from the compromises I had to make it composing and filtering - and perhaps since I'd never done anything like this before!
Again, very interesting discussion, and thanks for all the input and thought. It's nice to know that I'm thinking along the same lines regarding composition, even if the choice of crop I uploaded wasn't the best. I shall enjoy going back there when I have more time, and trying out various options!