This is probably a dumb question, but recently I've been trying to work out how Lupin was able to escape the Shrieking Shack every full moon. Because hear me out: according to the Harry Potter wiki (down in the trivia section of the Shrieking Shack page) it states that Dumbledore reinforced the structure to be abnormally durable, and placed protective enchantments on the Shack that prevented Lupin from getting out, for obvious reasons.
But then... he escapes every month to roam the grounds with James, Sirius, and Peter. And I know one of the explanations was that Peter as a rat was small enough to sneak up to the trunk of the Whomping Willow and stop it from fighting, but that still doesn't actually explain how Lupin was able to get out of the Shack itself.
And it wasn't like he wouldn't be taken to the Shack, because Dumbledore had no idea about the three Animagi or about Lupin escaping in the first place.
Is this just an oversight on my part, or a genuine PoA plothole?
Oh, thank you for the ask! đ
The situation you asked about is actually a bit complicated, but not impossible despite not being fully detailed in the books. Most of the hints towards understanding it are there though. Whether you consider this a plot hole or not is up to personal interpretation I think.
I've discussed this same issue before with someone else. The result we came to is that there must be a magical ward that keeps werewolves from passing it at the entrace hole at the Shack end, with kind of an on/off switch that can be disabled by non-werewolf persons. After all, a simple moving tree wouldn't stop a werewolf for very long, it would just have to move fast enough to avoid the branches or be reckless enough in its bloodlust to not care and off to the school grounds it would go. As well as the fact that it would be impractical to cast the protections every month! But they still have to get Lupin into the Shack somehow, as well as out. Logic dictates that this kind of protection with a switch should exist to make the Shrieking Shack situation possible, and it's clear that humans can pass through the wards anyway. Otherwise there would have been no point to plant the Whomping Willow specifically as extra protection to stop any humans from stumbling on the tunnel and wandering into the Shrieking Shack, the wards would handle that if humans couldn't pass through it.
"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."
(PoA, ch Moony, Wormtail, Padfoot, and Prongs)
Note the wording there. Not planted to keep him in, but to keep others out. Therefore, a special anti-werewolf ward must exist as well.
Furthermore, since werewolves don't attack animals (or animagi in animal form) the same way they do humans, it would have been safe for at least Sirius or Peter to go all the way to the Shack to disable the protections, let out Lupin, and then accompany him back all the way through the tunnel to the Hogwarts grounds, from where they could head out anywhere they wanted and had time for.
"They couldnât keep me company as humans, so they kept me company as animals," said Lupin. "A werewolf is only a danger to people. They sneaked out of the castle every month under Jamesâs Invisibility Cloak. They transformed ⊠Peter, as the smallest, could slip beneath the Willowâs attacking branches and touch the knot that freezes it. They would then slip down the tunnel and join me." [...] Soon we were leaving the Shrieking Shack and roaming the school grounds and the village by night. Sirius and James transformed into such large animals, they were able to keep a werewolf in check.
(PoA, ch Moony, Wormtail, Padfoot, and Prongs)
The way it's phrased the part where they all go to the Shack was back when they were only accompanying him in the Shack, and when they started roaming everywhere they presumably used the plan I sketched above - Lupin didn't say that they all came to the Shack every time after they started roaming, his words explicitly reference the time before they started doing it. Note that James Potter himself couldn't have walked the tunnel in his animagus form since he's a stag, which means he couldn't have participated in the release plan. The tunnel is cramped with a very low ceiling even for teenagers, he wouldn't have made it through it in animal form unlike all the others. He would have had to transform inside the Shack itself (with Sirius keeping Lupin at bay) whenever he went there.
[Harry and Hermione] moved as fast as they could, bent almost double; ahead of them, Crookshanksâs tail bobbed in and out of view.
(PoA, ch Cat, Rat, and Dog)
So there you have it! The answer is that they used the tunnel entrance to get out, which seems to only be protected with anti-werewolf enchantments that have a disabling switch that the others could operate since they're not werewolves.
(as for how Lupin could get inside the Shack as an adult..well, possibly they left the switch in an off position whenever he wasn't caged in there for full moon, or they were disabled after his graduation, or even that those specific protections are a kind that would wear off with enough time having passed. The complete non-reference to anti-werewolf wards is how this could qualify as an actual plot hole. But then again, the story isn't about the Marauders, it's about Harry Potter who has no need to know about the specific details of how they got Lupin out of the Shack twenty years ago, that info means nothing to him so it wouldn't make sense for them to explain it in detail. Which is how this can also be not a plot hole, just something completely irrelevant to the story in PoA!)