Sacred shrines built in honor of previous Avatars can be found in every nation. Many of these temples are kept in quiet places; far from the general public's domain, and are taken care of by monks and sages. The inner sanctuaries are often guarded by a large door that is unique to each temple. These doors require a large amount of bending to open; usually only a fully-realized Avatar can open them, or a handful of normal benders. The type of bending required to open a door depends on which nation the temple is built in; Water, Earth, Fire, or Air.
Avatar Roku's Temple in the Fire Nation