That's a good question. Back in 2005 one couldn't just ask AI powered google such things. So, my over-read about medieval castles brain was thinking that originally Hogwarts would have had an overhanging outdoor affair, at best, and most everyone would have used chamber pots emptied by the maid. So bathrooms would have been added much later as an afterthought, and since any bathroom would have been a wild luxury at that time, given the alternative was an outhouse or a chamber pot, having them anywhere inside would have been amazing. So they would have been positioned where the effluence could be easily piped to the moat (in this case, the lake). So, I was thinking, not in the common area of the house. Same problem by the way for the dungeons. Below lake level, no way to get the waste to flow to the lake, so those common rooms wouldn't either. I mean, other than magic. ;-) But architecturally, the castle seems like an ordinary castle.

I'm sure I overthought it. But that was the thinking.