Third Floor Corridor

The Third Floor Corridor is the most dangerous location in Hogwarts Castle.

Description
The Third Floor Corridor has a peculiar structure; after the body of the corridor comes a usually locked chamber, which contains a trapdoor. The trapdoor leads down into a series of chambers that, while not on the same level as the rest of it, are considered to themselves be part of the Corridor.

From 1991 onwards, the Third Floor Corridor was used by Professor Dumbledore as a guarding place for dangerous items (first the Philosopher's Stone, and later Lord Voldemort and his Horcruxes). The actual item would be stored in the last chamber, while the body of the corridor would remain empty and the series of chambers in-between would each contain a magical trial serving as protection.