Final answer:
Muggles see Hogwarts as an old ruin with danger signs, a result of enchantments to keep the magical world secret from the non-magical community.
Step-by-step explanation:
When Muggles look at Hogwarts, instead of seeing the grand castle that wizards and witches see, they perceive an old ruin with warning signs for danger and no trespassing. This is a magical enchantment placed on Hogwarts to keep it hidden from the non-magical community. The literary device used to describe this phenomenon is an example of how world-building is executed in the Harry Potter series. In the magical universe of Harry Potter, Muggles are kept unaware of the wizarding world through various spells and enchantments that make magical places and things unnoticeable or very unappealing to them.
These enchantments are a part of the series' rich lore and also demonstrate the lengths to which the magical community goes to protect its secrecy. This separation of the magical from the non-magical world is essential in maintaining the order and preventing chaos that might arise if Muggles were to discover the existence of magic. Hogwarts, as the principal wizarding school, is safeguarded not only by physical means but also by intricate spells that prevent revealing its true nature to those not privy to the secret magical world.