Later investigation has shown that these buildings were in many cases carefully planned and built fortifications. At Niekerk, for instance, nine or ten hills are fortified on concentric walls thirty to fifty feet in number, with a place for the village at the top. The buildings are forts, miniature citadels, and also workshops and cattle kraals. Iron implements and handsome pottery were found here, and close to the Zambesi there are extraordinary fortifications. Farther south at Inyanga there is less strong defense, and at Umtali there are no fortifications, showing that builders feared invasion from the north. These people worked in gold, silver, tin, copper, and bronze and made beautiful pottery. There is evidence of religious significance in the buildings, and what is called the temple was the royal residence and served as a sort of acropolis. The surrounding residences in the valley were evidently occupied by wealthy traders and were not fortified. Here the gold was received from surrounding districts and bartered with traders. As usual there have been repeated attempts to find an external and especially an Asiatic origin for this culture. So far, however, archeological research seems to confirm its African origin. The implements, weapons, and art are characteristically African and there is no evident connection with outside sources. How far back this civilization dates it is difficult to say, a great deal depending upon the dating of the iron age in South Africa. If it was the same as in the Mediterranean regions, the earliest limit was 1000 B.C.; it might, however, have been much earlier, especially if, as seems probable, the use of iron originated in Africa. On the other hand the culmination of this culture has been placed by some as late as the modern middle ages. What was it that overthrew this civilization? Undoubtedly the same sort of raids of barbarous warriors that we have known in our day. For instance, in 1570 there came upon the country of Mozambique, farther up the coast, "such an inundation of pagans that they could not be numbered. They came from that part of Monomotapa where is the great lake from which spring these great rivers. They left no other signs of the towns they passed but the heaps of ruins and the bones of inhabitants." So, too, it is told how the Zimbas came, "a strange people never before seen there, who, leaving their own country, traversed a great part of this Ethiopia like a scourge of God, destroying every living thing they came across. They were twenty thousand strong and marched without children or women," just as four hundred years later the Zulu impi marched. Again in 1602 a horde of people came from the interior called the Cabires, or cannibals. They entered the kingdom of Monomotapa, and the reigning king, being weak, was in great terror. Thus gradually the Monomotapa fell, and its power was scattered until the Kaffir-Zulu raids of our day.[31]