The Hogwarts passageways are confusing: We learn there is a passageway in the 4th floor of Hogwarts behind a mirror leading to Hogsmeade, but how can a passageway come out from 4 floors above the ground?
"Right into Hogsmeade," said Fred, tracing one of them with his finger. "There are seven in all. Now, Filch knows about these four" – he pointed them out – "but we're sure we're the only ones who know about these. Don't bother with the one behind the mirror on the fourth floor. We used it until last winter, but it's caved in – completely blocked." (POA chapter 10)
also the passageway behind the witch on the 3rd floor that leads into Honeydukes that Harry uses in the 3rd year – how is it possible?
"He turned, halfway along the third-floor corridor, to see Fred and George peering out at him from behind a statue of a humpbacked, one-eyed witch."
And finally, the passageway out of "the hog's head" (Aberforth's house), of the painting of Ariana Dumbledore is located on the second floor of the inn.
A possible answer can be that the Hogwarts walls are really wide, since the basilisk managed to travel through them, but that raises other questions – such as the castle would need to be twice the size, and just walking through the doorway would take a minute. also that still doesn't solve the problem, because even if the walls are really wide, there still isn't enough room for the passageway to descend, and there aren't any ladders in them.
I'm also pretty sure Hogwarts isn't connected to a mountain or something like that.