A trullo is essentially a large cylinder of very thick drystone wall, capped with a self-sustaining cone of calcareous slabs. The outer roof and the tip of the cone are often decorated with esoteric symbols of good luck. Originally the huts in which shepherds and farmers spent their nights away from home and in which they stored their tools and took refuge in case of inclement weather, real trulli are rather spartan. Very few ancient ones remain, as when a trullo is damaged, it is usually much easier and cheaper to tear it down and rebuild it using the original material than to repair it.
The thick stone walls are probably the first historical example of bio-building, as they soak up warmth during summer and release it in the cold season – doing exactly the opposite during summer. Their efficiency is so high that no other climate control is required. Another historical first is their modularity: families traditionally built an adjoining trullo whenever the need arose, then simply created an opening in the common wall. Finally, trulli also led the way for natural light design. Since the internal microclimate requires that they limit their openings to one door and one small window for air circulation, mirrored furniture is commonly used to reflect the sunlight coming in from the door throughout the trullo.
