The entrance is located behind a large painting of a Fat Lady in a pink silk dress. If you know the right password to tell her, she swings open to reveal a round opening in the wall. The common room contains a lot of squashy armchairs, a fireplace, and tables. The fireplace is connected to the Floo Network, but as it is extremely public except in the dead of night, Gryffindors usually use owls to communicate with their families instead. There's also a notice board, for public announcements such as the date of the next Hogsmeade weekend, but also for notices from individual students seeking to buy or sell secondhand books, or trade Chocolate Frog cards, or similar, as in any school.

Base of North Tower
Round room
access is by means of a ladder through a ceiling trapdoor from the room below
oppressive fire with a copper kettle that gives off sweet-smelling fumes
walls lined with shelves: tea cups, crystal balls, decks of cards, etc.
small round tables, squashy armchairs and little pouffes
at least one window which Harry opened just a crack to let fresh air into the room