The exact founding date of the city is unknown, but it has a long history. Kenyan school history books place the founding of Mombasa as 900 A.D. It must have been already a prosperous trading town in the 12th century, as the Arab geographer al-Idrisi mentions it in 1151. The oldest stone mosque in Mombasa, Mnara, was built 1300. The Mandhry Mosque, built in 1570, has a minaret that contains a regionally specific ogee arch. This suggests that Swahili architecture was an indigenous African product and disproves assertions that non-African Muslims brought stone architecture to the Swahili Coast.