The entrance to the Gryffindor Tower is located on the seventh floor of the castle, through the Portrait of the Fat Lady. If you tell her the correct password, the Portrait will swing open to reveal the round entrance to the Common Room. The room is filled with squashy armchairs and tables where the students study and socialize.
There are two staircases opening off the common room into the Tower itself: the boys’ staircase, on which the seven boys’ dormitories are located (one for each year) and the girls’ staircase, on which the seven girls’ dormitories are located (one for each year). The girls’ staircase is under a spell so that if a boy sets foot on it and persists for a few steps, a wailing klaxon goes off and the staircase temporarily Transfigures itself into a stone slide, sending him and anyone else on the stairs sailing to the bottom. According to Hogwarts: A History, the founders believed that girls were more trustworthy than boys, so that while the girls’ staircase is enchanted to prevent boys from getting in, the boys’ staircase does not carry a reciprocal enchantment (OP17). Each dormitory is a round room containing four-poster beds with windows looking out onto the grounds.