From Wikipedia: http://en.wikipedia.org/wiki/Old_Fort_of_Zanzibar

The Old Fort (Swahili: Ngome Kongwe), also known as the Arab Fort and by other names, is a fortification located in Stone Town, the capital of Zanzibar. It is the oldest building and a major visitor attraction of Stone Town. It is located on the main seafront, adjacent to another landmark building of the city, the House of Wonders (former palace of the Sultan of Zanzibar), and facing the Forodhani Gardens.