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In Harry Potter, a Fidelius Charm conceals a place by hiding knowledge of its location in the souls of one or more individuals. The location is "invisible, intangible, unplottable, and soundproof," according to Harry Potter Wiki. However, this does not prevent any knowledge whatsoever of the probable location of the concealed place, as evidenced by Death Eaters standing watch outside Grimmauld place when Harry Potter and Co. took up residence there.

The question is, does a Fidelius charm create any sort of protection from physical or magical harm? Could someone burn down a building protected thus, simply by setting a fire nearby? More relevant, could someone like Voldemort go nuclear and cast some sort of area spell that caught the concealed location in its range? It seems as if Voldemort could have done this, if the Death Eaters had some general idea of the location at which Harry, Ron, and Hermione were staying. There could have been additional protection spells so strong that Voldemort could not penetrate them, but wouldn't those nearly obviate the Fidelius Charm? (Picture Voldemort hovering impotently beyond the protective spells in Book 7).

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    I agree with the non-duplicity of this question, the other questions title is broad and may in fact cover many facets of the Fidelius Charm the description however clearly states what the author is interested in and the physical defense of the spell is not mentioned in anyway, either by question or answers. Furthermore this question now edited also presents a further aspect which is not covered in the other question about the mindset and motivation of the Death Eaters and why they for example didn't just go FiendFyre crazy on an area. – CandiedMango Oct 1 '15 at 5:31
  • The house was in the middle of a muggle village, which may have deterred extreme action to some extent (not that death eaters are known to care about the lives of the muggle masses) – user28875 Oct 1 '15 at 16:41
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    I have two options that occur to me. Does the location being 'intangible' make it impervious to such harm anyway? If not, there must be some physical protection to it or, in the situation you suggested, even fairly basic spells could be fired at the location to make life pretty awful and dangerous for the people inside. (I'm imagining small explosions fired at windows and doors, etc. Unlikely to kill, but would create a situation far different to life in Grimmauld Place). – ThruGog Oct 1 '15 at 21:44
  • While damage may occur from area effect spells it wouldn't do any good, there is no way of knowing your targets inside or if you penetrated any physical protection that might have been cast too. That's probably why deatheaters were watching the house. Still there's no point trying to kill someone if you don't know they are inside (otherwise they just charm another building): even if they are inside you cant see any shield spells that might be in place too (so you wouldn't know if you broke them). So physical damage is irrelevant as it would leave you uncertain of if you achieved your goal – Matt Sep 19 '17 at 20:03
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To shortly answer the question - objects protected by the Fidelius Charm are most probably vulnerable to "collateral damage" coming from the surroundings. However the Death Eaters either had no conclusive information or were restraining themselves from drastic actions even after taking over power.

More detailed - we see three times where the Fidelius charm is applied:

  • The Potters house - Voldemort already knew the secret so no need to apply mass destruction.
  • Grimmauld number 12 when Harry, Hermione and Ron were hiding there - Voldemort had no conclusive information about that. All he knew was that the house was deserted. He still left some guards to keep it under watch but they never noticed anything worth reporting (remember that he provides very thorough punishment for false alarms).
  • The Shell Cottage and The Burrow - as Neville mentioned - the Death Eaters try to spill as little pure blood as possible. They are rounding up muggle-borns because they have at least partial support from the community. An open attack on some pure blood family would have been a very unpopular move and there would be a risk of an open rebellion.

To answer the sub-question - does the Fidelius Charm provide physical protection from collateral damage - here some analysis:

Taking the description from Pottermore:

The Fidelius Charm is extremely ancient and still used to this day. It involves the concealment of information inside a living person. The chosen person, or Secret Keeper, is the only person who is thenceforth capable of revealing the protected information to others, however many previously knew it.

The main point of the Fidelius Charm is "concealment of information". No physical protection is mentioned.

Empirical evidence shows a bit different picture - there IS some kind of physical protection. The Death Eaters surely knew the location of Grimmauld number 12 as the house of the Black's family. They also knew that Sirius is the legal owner of the place as the last living descendant of the family. Nevertheless they were not able to access the place.

With all that I would call the Fidelius Charm "magical encryption".

In our world encryption includes having a secret key (consider it "the secret keeper") and applying some mathematics (known to the most people as "magic" :) ) so that certain information is converted into something completely unreadable. The one owning the secret key can easily read the information. For others it is EXTREMELY hard although not impossible to do so. In this case "extremely" means like 5-10-20+ years to break the encrypted data without owning the appropriate secret key. Encrypted data is not protected from destruction - everyone can freely damage or completely delete it so that it can no longer be recovered even via the secret key.

So replace:

  • "secret key" + "owner" -> "secret keeper"
  • "encrypted data" -> "hidden object"
  • "mathematics" -> "magic"

and you have the Fidelius Charm.

All in all - the Fidelius Charm is not a mean to withstand a siege. If cornered you would not be protected from indirect attacks. As an example - looks like the charm allows air to flow in and our of the protected building. Sirius lived for almost an year in the house and did not suffocate. So setting the surroundings of the protected building on fire would cause smoke to enter it and also eventually set it on fire. Using some mass destruction weapons will also destroy anything protected by the Fidelius Charm.

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  • Side note: other charms may have been placed on those locations to protect it from collateral damage. – neverendingqs Sep 29 '16 at 15:51
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As vap78 posted, the Fidelius Charm protects information, and not things. As Flitwick stated, if the protected secret is that the Potter family lives in a certain house, not even looking directly through the sitting-room window of that house, while the Potters are sitting in that sitting-room, will enable the observer to know that they live there.

This means that the knowledge of the Potters' residence cannot form in anyone's head, except by a deliberate revelation by the Secret-Keeper. The magic will cause even the most obvious conclusions, backed by all the evidence one could demand, to slip out of everyone's brain, leaving them clueless.

So the Potters' house was not directly protected from anything, but the mechanics of the spell stops anyone from seeing any point in attacking the house.

TL:DR: The Fidelius Charm makes you think, "These are not the droids we're looking for."

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