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. 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.