Located in a mountain valley at an altitude of 2200 m, Sana'a has been inhabited for more than 2500 years. In the 7th and 8th centuries the town became a major centre for the spread of Islam. This religious and political heritage can be seen in its 106 mosques, 12 hammams and 6500 houses, all built before the 11th century. Sana'a's many-storeyed tower or pisé houses add to the beauty of the site.