The Slytherin common room was located in the twisted dungeons. Which was a good idea for the past, when the castle was only built and, as He Xuan thinks, there were many who disagreed with the idea of magical school, as well as just personalities who were tempted by this land. But at this period of time, it’s not the most thoughtful idea: anyone, who may attack the castle, was likely to spend seven years of schooling here, so must know all the corridors. Another problem is that newcomers are unlikely to remember the way from the first try, that is why they will need a guide, which means even more interaction with the quite unfriendly Slytherin house. 