"It is a very cosy and welcoming place, as dissimilar as possible from Snape's dungeon. Lots of yellow hangings, and fat armchairs, and little underground tunnels leading to the dormitories, all of which have perfectly round doors, like barrel tops."
-- J.K. Rowling (BLC)
The Hufflepuff common room is round and has a low ceiling, somewhat like a badger’s home. There are little round windows at at ground level. As we have seen in the other common rooms, it is decorated in House colours, which in this case means that there are lots of yellow hangings; like Gryffindor Tower, it has a lot of comfortable armchairs. There are also many plants – some on wooden shelves along the walls and some hanging in plant holders with copper bottoms – and there are tables of polished light-colored wood. There is a mantelpiece carved with dancing badgers. Above this is a portrait of Helga Hufflepuff toasting her students with a small golden cup (Pm). There are “little underground tunnels leading to the dormitories, all of which have perfectly round doors, like barrel tops” (BLC, JKR).
Accessed from the the same corridor holding the painting leading to the Hogwarts kitchens: the rhythm of "Helga Hufflepuff" is tapped on the barrel two from the bottom, middle of the second row of a stack of big barrels past the painting and in a recess in the right hand corner of the corridor (Pm); to reach it, you enter a door to the right of the main staircase in the Entrance Hall and down a staircase there (GF17). If you tap the wrong barrel or do the wrong number of taps, you will be drenched in vinegar from one of the barrels (Pm).
The round doors to the dormitories are made of the same golden yellow wood as the tables in the common room (Pm).
Although it is at dungeon level, it is "as dissimilar as possible" from the Potions classroom (BLC).
Rowling originally planned for Harry to visit all four common rooms, but she was unable to work in a good reason for Harry to visit the Hufflepuff common room (Pm).
The details are reminiscent of hobbit architecture as described in J.R.R. Tolkien's book The Hobbit - MLW
From the Web
Ambient sounds of the Hufflepuff common room on Ambient-Mixer.com