Mecca or Makkah, is a city in the Hejaz in Saudi Arabia, the capital of its Makkah Region. As the birthplace of Muhammad and the site of Muhammad's first revelation of the Quran, Mecca is regarded as the holiest city in the religion of Islam and a pilgrimage to it known as the Hajj is obligatory for all able Muslims. The city is home to the Kaaba, by majority description Islam's holiest site, as well as being the direction of Muslim prayer. In its modern period, the city has seen tremendous expansion in size and infrastructure, home to structures such as the Abraj Al Bait, also known as the Makkah Royal Clock Tower Hotel, the world's fourth tallest building and the building with the third largest amount of floor area. During this expansion, the city has lost some historical structures and archaeological sites, such as the Ajyad Fortress. Today, more than 15 million Muslims visit Mecca annually, including several million during the few days of the Hajj. As a result, Mecca has become one of the most cosmopolitan cities in the Muslim world, despite the fact that non-Muslims are prohibited from entering the city.