The trullo is essentially a rural building type. With its thick walls and its inability to form multi-story structures, it is wasteful of ground space and consequently ill-suited to high density settlement. However, being constructed of small stones, it has a flexibility and adaptability of form which are most helpful in tight urban situations.
In the countryside, trullo domes were built singly or in groups of up to five, or sometimes in large farmyard clusters of a dozen or two dozen, but never for the occupancy of more than a single rural family.
The vast majority of trulli have one room under each conical roof, with additional living spaces in arched alcoves. Children would sleep in alcoves made in the wall with curtains hung in front.
A multiroomed trullo house has many cones representing a room each.
Along with its exterior wall, a trullo's interior room and vault intrados were often rendered with lime plaster and whitewashed for protection against drafts.
The thick stone walls and dome of the trullo, pleasantly cool in the summer, tend to become unpleasantly cold during the winter months, condensing the moisture given off by cooking and breathing and making it difficult to feel warm even in front of the fire. The inhabitants simply leave the doors open during the day to keep the interior dry, and live more outdoors than in.
Excerpt taken from Wikipedia.