The school is a thousand years old, so it was chartered ~ 900AD. I think it's more logical that the Founders retasked a defensive structure than that they would build a new open-plan campus. For one thing, it was a highly turbulent time. Wars (i.e., Catholic vs. Protestant,) battles between feuding lords, skirmishes with bandits, starting to hide from the muggles, and the old favorites like kidnap-for-ransom, made security for the children of the wealthy and influential a very important point.

I doubt that Hogwarts Castle was as glorious then as it is now. A thousand years of innovation in materials science, a thousand years of Charms masters and Headmasters each trying to leave their mark, population bubbles after war or plagues, and the incorporation of house-elves all undoubtedly contributed to major growth surges and beautification.

Does anyone really believe that the Castle had flush toilets and hot showers before *forks* were invented?

The ceiling in the Great Hall is undoubtedly some GENIUS's masterwork, possibly a lifelong project. Does anyone even want to *guess* at how much effort went into making the Room of Requirement? The mind boggles.

I've always assumed that the sentient suits of armor were designed as part of the castle's active defenses. The moving staircases might well have been conceived as ways to hinder or befuddle invaders, as well.

 

Touch activated vanishing charm and a warming charm

 

The Wizarding population... urk.

I can remember reading the "thousand" figure for the Great Hall, another that there's something like 100 students per house (400 overall), 100,000 or so at the Quidditch World Cup, the enormous Ministry of Magic, and then Rowling comes out and says the population of Britain is 3,000?

I figure the Wizarding population of Britain is somewhere between 50,000 - 100,000, similar to a mid-sized city.