Fluffy's chamber on the third floor didn't have any special lock. It could easily be unlocked using Alohomora.

Alohomora was a basic spell which even first years could use and that's how our heroes got into the Fluffy's chamber.

Instead of instructing students to not go into forbidden area on third floor, why wasn't the door simply locked with anti-Alohomora charm? It could also keep the thieves away.

Talking about feasibility of anti-Alohomora charm, in the quest to Philosopher's Stone, the trio found Alohomora not working on a door whose key was flying around. It means that doors can be locked in such way that Alohomora couldn't work.

Why wasn't the first door locked such way? Dumbledore himself could carry its key in his pocket.

marked as duplicate by user68762, JohnP, Edlothiad, Skooba, Dranon Jan 12 '18 at 17:16

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    Because Hagrid needed to feed Fluffy and he wouldn't have been able to open the door? – user13267 Jan 12 '18 at 15:09
  • The way this is a dupe with the linked question is it’s not the lock on Fluffy’s door that’s keeping people out. It’s Fluffy. – Edlothiad Jan 12 '18 at 16:06
  • @user13267 well, Hagrid would never have been able to open the door since to open it, he needed to perform magic, and he was banned from doing so. – Shreedhar Jan 12 '18 at 16:40
  • @Shreedhar Hagrid could open the door muggle way.. – Lobo Jan 12 '18 at 16:49
  • @Shree Hagrid can do basic magic (making the boat move when going to receive Harry), can partially to complex magic (giving Dudley a tail) and can get away with doing magic in the school (Dumbledore's protection) – user13267 Jan 13 '18 at 2:32

As @user13267 pointed out in comments, Hagrid needed to feed Fluffy and he wouldn't have been able to open the door if special lock was in place.

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