The First-Floor Girls' Toilets on the night the Troll destroyed it

The First-Floor Girls' Toilets is where Harry, Ron and Hermione first become friends. The boys save Hermione from a Mountain Troll here.

From the Story

Harry and Ron save Hermione from a mountain troll in the castle’s first-floor girls’ toilet. The pair, intending on cornering the creature, accidentally trap Hermione in the bathroom with it. When they realize what they have done and rush in to save her, the trio work together to knock the troll out with its own club, and become friends from that moment forward.