This is an add-on Secret Cavern which you should be able to add to any room you already own. This model uses an elegantly styled Book Shelf as the default level room item, and when you click it, your Avatar is automatically transported to the lower level. P.S. If you find it inconvenient move the furniture back to the lower levels before you invite your friends. This is beyond my control and I guess we'll have to wait for the programmers to find a solution.