Why wasn’t the Whomping Willow passage guarded?

Question

By the end of PoA, we find out that Lupin, Snape, and Dumbledore all knew about the passage under the Whomping Willow the whole time. So why was it not guarded (by Dementors or something/one else) since this seems like an obvious entry into Hogwarts for Black?

Update: Sorry this wasn't clear when I originally asked, but I thought it was obvious that the Whomping Willow itself was not an effective guard against Black. He already knew how to use the knob to disarm it, which we know can be done by a large stick for those going out and easily reached by those coming in.

Answer 1

I'm going out on a limb here.

Because Dumbledore thought that Black was innocent.

The ministry had the Dementors as guards, which Dumbledore was most unhappy about but suffered outside the school grounds. He would not want them inside the school.

But Dumbledore knew about the passage, and that Black could get past the guardian (the Whomping Willow). If he had doubts about Black's innocence he could have blocked the tunnel.

I'm not quite sure when Dumbledore changed his mind about Black. It seems he was convinced of his guilt enough initially to testify against him. But by the end of the book (PoA), he is sure of his innocence. Of course, you never know quite what Dumbledore is thinking, I wouldn't put him past him to get someone falsely imprisoned for years as part of some long plan.

From memory, it is always people from the Ministry or other teachers who warn Harry about Black, Dumbledore seems more worried about the Dementors.

edit with some additional thoughts

While Dumbledore may not have been 100% sure of Sirius' innocence, he must have been interested enough to let it play out. It is quite possible that he testified against Sirius when he was stricken with grief. There are some oddities that may have made him think twice about it.

• Sirius had the ability to attack Harry while in Godric's Hollow instead, he lent Hagrid his motorbike.
• Why did he wait all those years before he tied to escape, it would have been much easier to attack harry before he arrived at Hogwarts (we assume he would not know of the blood protection)?

Dumbledore has always been shown to be very contemplative, so it is quite possible that as the grief of the Potters' death subsided, his certainty of Black's guilt would have too.

He has also been shown to be happy to put people at risk to find answers, leaving a path for Black to get into Hogwarts, and employing a previous friend. Could have been an attempt to unravel the mystery of what happened.

Answer 2

The Whomping Willow was the method of guarding the passage.

When the tunnel was originally built, to sneak Lupin in and out of Hogwarts, the Whomping Willow was put there specifically to guard that passageway.

“I told you, months ago, that the Whomping Willow was planted the year I came to Hogwarts. The truth is that it was planted because I had come to Hogwarts. This house –’ Lupin looked miserably around the room, ‘– the tunnel that leads to it – they were built for my use. Once a month, I was smuggled out of the castle, into this place, to transform. The tree was placed at the tunnel mouth to stop anyone coming across me while I was dangerous.”
_{^{- Harry Potter and the Prisoner of Azkaban, Chapter 18 (Moony, Wormtail, Padfoot and Prongs)}}

Anyone coming from the Shrieking Shack would still encounter the Whomping Willow, so it would be an effective guard from anyone trying to break into Hogwarts as well. The tree can hit quite hard, and there’s only one way to get past it - by pressing a specific knot on the trunk. Even if someone knows how to stop the tree, they still would have to physically do it without being hit first. When Harry and Hermione were near it, it did quite a bit of damage to them.

“Harry – we’ve got to go for help –’ Hermione cried; she was bleeding, too; the Willow had cut her across the shoulder. ‘No! That thing’s big enough to eat him, we haven’t got time –’

‘We’re never going to get through without help –’

Another branch whipped down at them, twigs clenched like knuckles.”
_{^{- Harry Potter and the Prisoner of Azkaban, Chapter 17 (Cat, Rat, and Dog)}}

For a person to stop it, they’d most likely need a long stick, since the tree probably wouldn’t let them get close to the knot.

“Sirius thought it would be – er – amusing, to tell Snape all he had to do was prod the knot on the tree-trunk with a long stick, and he’d be able to get in after me.”
_{^{- Harry Potter and the Prisoner of Azkaban, Chapter 18 (Moony, Wormtail, Padfoot and Prongs)}}

It’s possible that if they did think of the passageway to the Shrieking Shack, the staff considered the Whomping Willow to be enough protection for it on its own. Lupin was the only one who knew Sirius was an Animagus, and he was keeping it a secret from everyone including Dumbledore, so they’d presume it would be a human they’d be dealing with, and wouldn’t know he could turn into a dog that was smaller and lower to the ground.

Lupin wouldn’t have told Dumbledore that Sirius could get past it.

Lupin did know that Sirius could get past the Whomping Willow, but he was very much keeping it a secret from everyone else that Sirius was an Animagus and was with him every month. He thought about telling Dumbledore after finding out Sirius escaped from Azkaban, but he never actually did.

“Lupin’s face had hardened, and there was self-disgust in his voice. ‘All this year, I have been battling with myself, wondering whether I should tell Dumbledore that Sirius was an Animagus. But I didn’t do it. Why? Because I was too cowardly. It would have meant admitting that I’d betrayed his trust while I was at school, admitting that I’d led others along with me … and Dumbledore’s trust has meant everything to me. He let me into Hogwarts as a boy, and he gave me a job, when I have been shunned all my adult life, unable to find paid work because of what I am. And so I convinced myself that Sirius was getting into the school using Dark Arts he learnt from Voldemort, that being an Animagus had nothing to do with it … so, in a way, Snape’s been right about me all along.”
_{^{- Harry Potter and the Prisoner of Azkaban, Chapter 18 (Moony, Wormtail, Padfoot and Prongs)}}

He also therefore wouldn't have told Dumbledore that Sirius knew how to get past the Whomping Willow, since that would reveal that he was breaking the rules.

“I sometimes felt guilty about betraying Dumbledore’s trust, of course … he had admitted me to Hogwarts when no other Headmaster would have done so, and he had no idea I was breaking the rules he had set down for my own and others’ safety. He never knew I had led three fellow students into becoming Animagi illegally.”
_{^{- Harry Potter and the Prisoner of Azkaban, Chapter 18 (Moony, Wormtail, Padfoot and Prongs)}}

Lupin wouldn’t have told Dumbledore that Sirius could get past the Whomping Willow, because explaining that would also mean having to admit he’d betrayed Dumbledore’s trust and broken the rules.

Answer 3

There was no Hogsmeade entrance to the Shrieking Shack.

The only way in or out was via the secret passage from Hogwarts, past the Whomping Willow, and presumably locked from the Hogwarts side as well, though perhaps it was left unlocked when Lupin was no longer in residence.

The Shack would likely have been magically protected so as to protect any would-be intruders from Lupin, and this would have included some sort of magical alarm to alert Dumbledore if someone was attempting to break in. If not, Dumbledore would certainly have taken steps after Black escaped from Azkaban.

... and, of course, as already mentioned, any intruder would also have to get past the Whomping Willow. Given that Black knew the secret to immobilizing it, however, I doubt that Dumbledore would have been depending entirely on that to keep him out.

Answer 4

Whomping Willow did not need any guarding because the only way to get through the passage would be to immobilise it by touching the knot on the trunk of the tree.

Crookshanks darted forward. He slithered between the battering branches like a snake and placed his front paws upon a knot on the trunk. Abruptly, as though the tree had been turned to marble, it stopped moving. Not a leaf twitched or shook.
_{Harry Potter and the Prisoner of Azkaban, Chapter 17}

A werewolf is only a danger to people. They sneaked out of the castle every month under James’s Invisibility Cloak. They transformed... Peter, as the smallest, could slip beneath the Willow’s attacking branches and touch the knot that freezes it. They would then slip down the tunnel and join me. Under their influence, I became less dangerous. My body was still wolfish, but my mind seemed to become less so while I was with them.
_{Harry Potter and the Prisoner of Azkaban, Chapter 18}

It's obvious that one could immobilise the tree only from the outside and not while coming from the Shrieking Shack. So, we can assume with a fair amount of confidence that whoever knew about the path did not think it was passable.