Castle Secrets
We all have our secrets. Some more than others, but we do have that in common. And as it turns out, sometimes locations are no exception to this, either — they have their own secrets as well.
Room of Requirement
The Room of Requirement, also known as the "Come and Go Room," is located on the seventh floor of the castle. You may not have noticed it, and there is a possibility you will never see it, because it only appears to people under specific conditions. It's always where it should be, though it only appears when people really need it — once it appears it will always be equipped with what we're in the need of.
However, the room's magic does have limitations, and those usually follow Gamp's Law of Elemental Transfiguration. So you can't really use the room for becoming richer, getting yourself food, etc. Furthermore, it is advised that people think through their needs properly and focus hard on the specifics of their wish while walking three times past the blank wall. If you follow those instructions and you're truly in need, the Room of Requirement will be there for you.
Philosopher's Stone Room
The Philosopher's Stone was a thing of legends. It was a one-of-a-kind substance created by Nicholas Flamel and was located in the castle until the Dark Lord tried to steal it, after which it was destroyed in order to prevent it from falling into the wrong hands. It is rumored it can turn metal to gold, and that with its help you can create the Elixir of Life, which gives the drinker immortality.
The entrance to this location starts at the castle's third floor, through a trap door guarded by a huge three-headed dog that has to be put to sleep by playing some music to it. Down through the trap door one falls right into a chamber full with Devil's Snare, and you need to create light or fire in order to get rid of it, or you'll get strangled to death. In the next chamber there are thousands of flying keys and one needs to use a flying broomstick to catch the correct key while running away from the rest of the keys that attack you once you're on the broom. Once you have the key, you can enter a chamber that is literally a chess board, and you have to play your way through the board by winning a game of wizarding chess against living statues.
The chamber after that has a Mountain Troll in it and you must defeat it in order to progress further. Once you enter the next room, both the paths ahead of you and behind you become closed off with flames. To move forward you complete a puzzle — you need to choose the correct two potions out of seven with the help of a riddle if you want to be able to cross the flames, otherwise you'd burn if you attempt to go through them.
The last chamber simply has the Mirror of Erised in it. The trick is that when looking into the mirror one needs to see himself or herself as having the stone but not using it, if they want to obtain it. If you see yourself as using the stone, it will not materialize/appear for you.
Chamber of Secrets
As we already mentioned in a previous lesson, to reach the Chamber of Secrets you have to go through Moaning Myrtle's bathroom. The room was created in secret way back when the school was first founded, by Salazar Slytherin, as a result of the difference in opinions between him and the other three founders.
Salazar Slytherin believed that the founders should be more selective and elitist about the students they take into Hogwarts and he believed only pure-bloods should be allowed to study magic there. When the other founders disagreed, he created this room that holds a very dangerous and lethal monster — a Basilisk. He protected the chamber the best he could so that only his rightful heir would be able to open it and unleash the Basilisk to purge the school of all muggle-borns and half-bloods. This also ensured that no one could randomly stumble upon the chamber, and the other founders or school staff could not locate it either.
Presumably when one falls down from the passage beneath the sink with the snake-engraved tap in Myrtle's bathroom, they find themselves in a corridor at the end of which there is another door that can only be open with the use of Parseltongue. For those unaware, that is a snake language. Beyond that, there are huge pipe-like tunnels that lead to a chamber whose floor is covered with water. The ceiling is so high one can barely see it, it is supported by huge stone pillars entwined with carved serpents, and in the depths of the room there is a gigantic statue of Salazar Slytherin that towers as high as the chamber itself. The statue is where the Basilisk is waiting patiently and dormant, until it receives commands in Parseltongue that will unleash it to the rest of the school.
Secret Passageways
According to a certain magical map called "The Marauder's Map" and other trustful sources, even though the castle is unplottable (meaning no one can apparate or disapparate inside it), there are some secret paths that lead in and out of the castle. The Marauder's Map was created by Moony (Remus Lupin), Wormtail (Peter Pettigrew), Padfoot (Sirius Black), and Prongs (James Potter). It's a magical map that not only shows every last bit of the castle, including secret chambers and passageways, but it also displays the presence and movement of every person inside the castle (no matter if they're using magic to disguise or conceal themselves).
Whomping Willow—Shrieking Shack
A tunnel under the Whomping Willow leads directly into the famous "horror house" known as the Shrieking Shack, which no one visits because of all the scary stories about it. Those stories were started as rumors for the sole reason of keep people away from the place. In order to pass one must first paralyze the Whomping Willow or otherwise press a certain knot at the base of the tree. This will let you go through without the Whomping Willow smashing you to pieces.
Gunhilda of Gorsemoor (hump-backed witch statue)—Honeydukes Sweet Shop in Hogsmeade
This statue is located halfway through a corridor on the third floor and under its hump is a secret entrance to a passageway that leads straight into the Honeydukes Sweet Shop in the Hogsmeade village. One needs to tap the hump with their wand and say "Dissendium!" This sounds like a great shortcut for getting some sweets.
Gregory the Smarmy—Hogsmeade
Behind the statue of Gregory the Smarmy lies the entrance to a passageway that leads to Hogsmeade, though it is unknown where exactly does this passage take you to specifically. We'd advise against using it, just in case you end up in a unpleasant location.
Mirror—Hogsmeade
A certain mirror on the fourth floor hides a passageway to Hogsmeade, though it has now been caved in. In the past it was very wide and according to certain people it was big enough for holding a meeting for an organization.
Vanishing Cabinet (Hogwarts)—Vanishing Cabinet (Borgin and Burkes, Knockturn Alley)
There exists a vanishing cabinet in the castle which is directly linked with another vanishing cabinet that is located in the Borgin and Burkes shop in Knockturn Alley. Anyone, or anything, going inside one cabinet will be sent to the other cabinet.
Room of Requirements—Hog's Head and More
Since the Room of Requirement changes based on the people's needs, it is possible that there could exist more than one passage and that all of them lead to a different location. But the one that we know for certain exists leads to the famous Hog's Head bar. Some clearly needed a drink.