Thanks to Joel/Charely Potter

From the start of the spiral staircase branching off from the common room are the girl's dormitories. First years are allocated the dormitories down the bottom, and the higher you climb, the older the residents are. This is so that seventh years, who are roomed up the very top, have a little less noise reaching their dorm so that they may study in quiet or rest well in preparation for exams. There are 2+ dorm rooms for each year.

Every dorm room is equipped with a centrally-located stove, to keep you all warm. Each of you will have your own four poster bed, flanked by a set of windows and space for storing personal belongings, and a mirror in case you need to watch yourself groom. On the window sill is a jug of water and a cup, supplied and freshened daily by the elves. There is a place for your trunk and individual wardrobes to hang your clothing. Space isn't an issue, thanks to the many charms.

MALES BEWARE! You may not enter the girl's dormitories. Even though Professor Newton can't be there to physically stop you, if you TRY to climb the stairs they will turn to a slide and you will tumble to the bottom.