Why was the entrance to the Stone's dungeon hideout located on the third floor?

Question

I've been reading the Harry Potter books for years and there's something I noticed the other day which I can't believe I hadn't spotted before.

The Philosopher's Stone is hidden in some dungeons beneath Hogwarts. This is confirmed by Dumbledore.

"What happened down in the dungeons between you and Professor Quirrell is a complete secret, so, naturally, the whole school knows."
_{(Philosopher's Stone, Chapter 17, The Man With Two Faces).}

Hermione also comments on how far the drop is after they fall through the trap door.

"We must be miles under the school," she said.
_{(Philosopher's Stone, Chapter 16, Through the Trapdoor).}

So the Stone is kept in the dungeons. But the entrance is on the third floor. How does that make any sense?

The fall from the trapdoor is clearly not meant to kill or harm the intruder. So why not have the entrance in the dungeons themselves - or at least on the ground floor?

Surely that means a lot of architectural effort in rejigging Hogwarts. Harry, Hermione and Ron start off on the third floor and end up falling through the second, first and ground floors. Did they have to insert this chute into the middle of their ancient castle? Was it there by design? Or are we to suppose that this huge funnel running through the core of the building and a series of connected underground chambers were part of Hogwarts all along? It's a happy accident, if so.

Answer 1

We don't really know as it is not stated in the books. But here is my educated guess: We learn from Chamber of Secrets, that there are hidden places within Hogwarts that are only accessible from certain places. I don't think it is unlikely that there are more hidden places like that.

Take the Room of Requirements as an example: Its entry is in the seventh floor and still there is a secret passage to Hogsmeade...

I think that the dungeons had been there forever and they happened to be accessible from that trapdoor on the third floor.

Their single -hidden- entrance makes them predestined for the task of hiding the Philosophers Stone.

So I guess: They did not build the place to hide the stone originally but rather chose the best place they had and made it even harder to access with their traps / spells.

Answer 2

I don't think rearranging Hogwarts would be such a hard task, given that the castle is able to rearrange itself (for example staircases are reportet to lead to different places at different times). I coul imagine the headmaster having some control over it.

That leaves the question why they decided to do so. Well, I can see three main advantages of the 3rd floor vs. the dungeons:

• It's in a more populated part of the castle, so breaking in is more likely to raise suspicion. The dungeons are mostly used for potions classes, Snapes office, the Slytherin common room and that's it. The 3rd floor is home to classrooms and would be regularly crossed by students from differnt houses on their way to class (Slytherins and Hufflepuffs for classes >2nd floor, Gryffindors and Ravenclaws for classes in the grounds or <4th floor).
• Most teachers offices aren't in the dungeons. More teacher offices and more classrooms nearby means higher chances of a teacher present should something happen.
• You also have to consider the houses: Would a Slytherin report another Slytherin, schould they try to break in? Propably not, all they'll have to gain is loosing points for their own house. Would they report a Gryffindor? Sure! Why is that important? The third floor is pretty much the middle ground between the houses, Gryffindor and Ravenclaw being located in towers, Hufflepuff near the kitchens (so below ground level), Slytherin somewhat deeper in the dungeons - house loyality impact on reporting of suspicious behavior is minimized.