AN: Enjoy this glorious piece of artwork!
Hogwarts; A History. Ed XI. Excerpt from Chp 12; Founders. Pg 294-326.
…Salazar Slytherin was not the only one of the Founders to leave a legacy for the students to find. While Slytherin left a secret chamber with all of his creatures and notes on Dark Magic, it could only be found by those with the gift of Parseltongue. His chamber was only found in 1992, by Harry Potter and his friends (See footnote).
Rowena Ravenclaw left something not that much different from Slytherin. She left a hidden room, attached to her house's common room, filled with self-updating bookshelves. Whenever a book is published by a Hogwarts Alumni, a copy will appear on a bookshelf in the room, shelved correctly. This Raven Library, as it was named, was stumbled across in the early 1940s, and can only be accessed by Ravenclaw students.
Helga Hufflepuff's legacy was rather extensive and can be accessed by those in her house. She created several hidden chambers connected together and attached to the Hufflepuff common room. Huffle Set, as they were called, are extensive enough to include greenhouses, an indoor Quidditch Pitch, passages to the kitchen, library, and study rooms and games rooms. The Set was only found in 2000 AD.
The fourth founder, Godric Gryffindor, left something vastly different for the students in his house. As a Knight, he felt that something practical was needed to be passed down. He left an armoury and a large training hall. These could be accessed through an alternate password given to the common room guardian. This was discovered purely by accident by Harry Potter, during the Dark Lord Voldemort's second rising (See footnote).
Why the Founders chose to hide these legacies from the school, is a question no one can answer. The better question would be, why did it take us so long to find these legacies? It had taken us around a thousand years to find these. And yet, we're the ones chosen by the Founders…
Footnote: See Harry Potter; The Boy-Who-Lived.