Just an odd thought about something Draco said in Chapter 48:

Before them was a small empty place of stone set against the night sky. Not a roof like the one he'd dropped Harry from, but a tiny and proper courtyard, far above the ground. With proper railings, elaborate traceries of stone that flushed seamlessly into the stone floor... How so much artistry had been infused into the creation of Hogwarts was something that still awed Draco every time he thought about it. There must have been some way to do it all at once, no one could have detailed so much piece by piece, the castle changed and every new piece was like that. It was so far beyond the wizardry of these fading days that no one would have believed it if they hadn't seen the proof in Hogwarts itself.

...is - is Hogwarts sentient? If it's animate, capable of creative expression, and self-constructing, it's not out of the question that Hogwarts might be in some sense intelligent or alive. It'd also explain some things about the Hogwarts security system, to say nothing about the Room of Requirement, in canon.