Headcanon: Like Salazar Slytherin, each founder had a hidden chamber. Godric Gryffindor made a training room full of magical weapons, only to be opened by a true Gryffindor. Rowena Ravenclaw made a library full of every book ever written, muggle and magical, updating every day, only to be opened by those who trukly crave knowledge. Helga Hufflepuff made a simple room, made to cater to the needs of anyone who needed it. To this day, it is known as the Room of Requirement.