It is unclear when Bantu-speaking peoples first moved into the country from the north, although AD 600 seems to be a consensus estimate. In that era, the ancestors of the modern-day Kalanga moved into what is now the north-eastern areas of the country. These proto-Kalanga were closely connected to states in Zimbabwe as well as to the Mapungubwe state and the notable of these was Domboshaba ruins which is a cultural and heritage site in Botswana originally occupied towards the end of the Great Zimbabwe period (1250–1450 AD) with stone walls that have an average height of 1.8 metres. The site is a respected place for the people living in the region and it is believed that the chief lived on the top of the hill together with his helpers or assistants. These states, located outside of current Botswana's borders, appear to have kept massive cattle herds in what is now the Central District—apparently at numbers approaching modern cattle density. This massive cattle-raising complex prospered until 1300 AD or so, and seems to have regressed following the collapse of Mapungubwe. During this era, the first Tswana-speaking groups, the Bakgalagadi, moved into the southern areas of the Kalahari. All these various peoples were connected to trade routes that ran via the Limpopo River to the Indian Ocean, and trade goods from Asia such as beads made their way to Botswana most likely in exchange for ivory, gold, and rhinoceros horn.