An old man walks along a street in Old Sana'a. Surrounded by a massive 20 to 30-foot high wall, old Sana'a is one of the world's oldest inhabited cities. Several stone and brick buildings date back to the 11th Century AD; many are over 400 years old. Modern buildings follow the same 1,000-year-old distinctive Yemeni architectural style. Natural dressed stone is used to construct the lower floors and fired bricks on the upper floors. Exterior walls are beautifully ornamented with plasterwork friezes. Circular and arched windows are richly decorated too.In 1986, the city was designated a World Heritage Site by UNESCO after work had begun to restore many fine but neglected buildings.