I have heard that the castle on the cover is based on Lochranza in Arran, whether visited by Herge in person I can't remember.
Lochranza has certainly been cited, and seems appropriate, but the fact is it has to be pointed out that other than being a castle on an island, it isn't really the same shape as the castle which Tintin visits; it's a squarish, boxy thing, without the round towers of Hergé's book.
It's true that Bob De Moor (Hergé's assistant and colleague) did
make a trip there as part of his researches for the update to the book in the sixties, but while he used the topography and scenery to enhance the book, he took more from Broddick Castle for the re-vamp of the buildings on the island.
In the original version (in black and white) which still served as the template for the castle even after the re-drawing, Hergé seems to have taken a magpie approach to the castle he drew, with several sources, including the curious double topped tower which surmounts the castle from an engraving of Arundel Castle in England (which is still in his files) and possibly using Château Beersel in the Brabant for the stairs and corridors of the interior.