The large corridors can probably be explained by magic, just like the Weasley home's lack of structural integrity by conventional standards. Large windows could have been retrofitted in much the same way, and it would be shocking if the huge and sprawling modern facility had not been a product of repeated expansions and additions. THe only rooms I recall being definitely mentioned as products of the founders are the Great Hall and Chamber of Secrets - and the Chamber is a similarly large and open space (and part of it notably collapses at one point when Gilderoy fires off a malfunctioning spell with Ron's damaged wand) and the Great Hall is never described as having windows at all, only being lit by floating candles and the enchanted ceiling.
What is the history of the actual castle used for filming the movies? JKR sketched her own map well before they were made (I can't find the image, but I know I saved a copy somewhere when I found it) but many such fortifications were built and expanded and rebuilt in the same location for a very long time since the factors that make a p[articular place a good one for a fort didn't significant;y change until very recently in historical terms.
What is the history of the actual castle used for filming the movies? JKR sketched her own map well before they were made (I can't find the image, but I know I saved a copy somewhere when I found it) but many such fortifications were built and expanded and rebuilt in the same location for a very long time since the factors that make a p[articular place a good one for a fort didn't significant;y change until very recently in historical terms.
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‎noli esse culus
‎noli esse culus