Your point about landscaping is an interesting one, but I'm not sure I agree. Sure, when I think 'Classic-Castle', I usually think of sets that are just a castle, or just a cart, or just a piece of wall:
But there are a lot of instances that include a little bit of landscaping, either incorporated into the main structure, or just a little side add-on of a tree, bush or rock, even going back to 383. Of course this is most prominent in the Forestmen and Dark Forest subthemes, but elsewhere as well:
I think your much stronger point (along with the yellow vs fleshy distinction) is the fact that Classic-Castle tends to generic figures while licensed themes are all about collecting them all to get all of the key characters. Of course, the problem of getting an army of Luke Skywalkers and and army of Harry Potters is also seen in an army of King Leo's etc, and an army of Jaykos or Danjus is so much worse.
You also have a great point about the widely varying definitions of 'castle'. If we limit ourselves by time period, we are faced with the fact that in the same year, some areas might be enjoying the full Renaissance, while others are still (socially, technologically) very much in a high middle ages mode, while for peasants in outlying areas there might be no real difference from what they would have seen in the early dark ages era. Add to that the fact that any fantasy literature is going to be not in our earthly time-scale at all (yes, I know, Tolkien is in some mythical pre-history era), so where do you put it? Roughly I think we're left with a technological distinction, and so your 'stones and swords but not gunpowder' definition is where we're left. There is also the problem of realism vs magic, but at least in terms of 'Classic-Castle' we have Majisto, Willa*, and dragons, so we can't just reject anything fantastic as 'not Castle' (*yes, I know, Willa may be on the wrong side of many people's cutoff for 'classic-castle').
So if we come down to 'stones and swords but not gunpowder' and magic is okay, we would have to group Tolkien stuff along with castle. If it comes down to fig skin tone, it is out, along with any licensed theme.
I think what we need to do is start talking about official sets along the lines of 'castle theme' and 'castle-friendly themes'. So any licensed themes, or selected things like a few of the adventurers sets, or selected Collectible figs, or things like the Creator dragon, fall in the 'castle-friendly' category.