What are some distinctive features of a hobbit hole?

A hobbit-hole, also called smial, is a from of housing used by the hobbits in J.R.R. Tolkien's fiction. It is basically an underground house built in hills, consisting of a long hallway with the rooms branching off to both sides; usually the hallway runs parallel to the shape of the hill so that the rooms on one side of the hallway can have Windows. Further characteristics of hobbit architecture are perfectly round doors and Windows, tube shaped halls, and building even-floored (meaning no housings with multiple floors/stories). Most hobbits do not live in hobbit-holes but in "normal" houses, which have the same architectural characteristics as The Hobbit-holes mentioned above.