There are hundreds of disparate Na'vi clans on Pandora. Some of the clans, including the Omaticaya, live in ancient trees that are two to three times the height of the redwoods that once were seen in the Pacific Northwest on Earth. The circumference of Hometree is great enough to house hundreds of clan members. The tree has a number of radially arrayed natural hollows, supported by columns which themselves are the size of redwood trunks. (Read more)
There are four levels of these vaulted spaces which are utilized by the Na’vi: the lowest level, which is a subterranean catacomb amongst the roots of the tree, is reserved for the most sacred rituals of the clan, including Uniltaron. The “ground floor” level becomes the village commons, in inhabited Great Trees, while the third level (second above ground) is used for communal eating, meetings and some ritual gatherings. The fourth level is an open-work of struts and spokes that is used for sleeping. The Na’vi string their hammocks (or more accurately “cultivate” them, since they are living epiphytes) far above the ground and away from large predators who sometimes prowl among the lower columns at night. Above that, the radial spokes continue upward, through the hollow core of the trunk, for over 100 meters until the first of the tree’s massive limbs, where the core narrows into solidity.

Geological surveys have identified a large deposit of unobtainium directly beneath the Hometree of the Omaticaya clan, which puts its long term habitability in question.