"The buildings on Mozambique Island were the oldest buildings I had ever seen, except for Great Zimbabwe of course, but that was a ruin."
Mozambique Island
Creative Commons AttributionMozambique Island - Credit: stignygaard

The fortified Ilha de Moçambique, now a Unesco World Heritage site,  is a former Portuguese trading-post on the route to India. The island was a major Arab port (the name comes from one-time resident Musa Al Big) and boatbuilding centre long before Vasco da Gama visited in 1498. The Portuguese established a port and naval base as early as 1507.  The Chapel of Nossa Senhora de Baluarte, built in 1522, is considered the oldest European building in the southern hemisphere.

 

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Great Zimbabwe
Creative Commons AttributionGreat Zimbabwe - Credit: ctsnow, Flickr
The ruins of Great Zimbabwe, after which the country is now named, are thought to date from the 12th century.  Zimbabwe means "stone buildings" in Shona.

Great Zimbabwe is near Masvingo, in Southeastern Zimbabwe.