The Slytherin Common Room (in the dungeons, under the lake) as I imagine it might have looked like during the time of the Marauders & Snape.
About this creation
The Slytherin Common room has very low ceilings, but you'll still see Severus Snape sitting by himself in the back at a table with potions and his potions book. The wall on the side with a snake on it swings out for entry. The cut-outs let you see a little of what's in the lake above. And on the top there is Lily Evans in a boat, playing with the Giant Squid.