The entrance to the Slytherin common room is concealed behind a stone wall in the dungeons of Hogwarts Castle. The door is magically sealed and can only be opened by speaking the correct password. The password changes every month, and only the Head of Slytherin House knows the current password.
There are several ways to gain access to the Slytherin common room if you do not know the password. One way is to use a Polyjuice Potion to transform yourself into a Slytherin student. Another way is to use a Summoning Charm to summon the door open. However, the most common way to gain access to the Slytherin common room is to simply ask a Slytherin student to let you in.
