Great Zimbabwe was an important capital city of the Bantu-speaking Shona people between the twelfth and fifteenth centuries, reaching its peak between 1250 and 1450 with an estimated population of approximately fifteen thousand people and control over a large territory. Covering an area of nearly two thousand acres, the ruined city of Great Zimbabwe is principally comprised of three structures: the Hill Complex, the Valley Ruins, and the Great Enclosure, which are surrounded by a large protective wall, nearly thirty feet tall. The Great Enclosure, which dates from the mid-fourteenth to fifteenth century, was made with a special pattern of dry stone blocks, a technique still used by contemporary builders, and is the largest stone structure in sub-Saharan Africa. Many sculpture and pottery fragments have been found at the Great Zimbabwe site, indicating a rich art culture. A popular material for sculpture was soapstone, and many examples of soapstone bird carvings have been discovered, though the exact significance of these sculptures is still unknown.