Azerbaijan, the nation and former Soviet republic, is bounded by the Caspian Sea and Caucasus Mountains, which span Asia and Europe. Its capital, Baku, is famed for its medieval walled Inner City. Within the Inner City lies the Palace of the Shirvanshahs, a royal retreat dating to the 15th century, and the centuries-old stone Maiden Tower, which dominates the city skyline. The Azerbaijan Democratic Republic proclaimed its independence in 1918 and became the first democratic state in the Muslim orient world. The country was incorporated into the Soviet Union in 1920 as the Azerbaijan Soviet Socialist Republic. After the fall of Communist rule it was not until the 2000s that the tourism industry began to recover, and the country has since experienced a high rate of growth in the number of tourist visits and overnight stays. In the recent years, Azerbaijan has also become a popular destination for religious, spa, and health care tourism. During winter, the Shahdag Mountain Resort offers skiing with state of the art facilities. The government of Azerbaijan has set the development of Azerbaijan as an elite tourist destination as a top priority. It is a national strategy to make tourism a major, if not the single largest, contributor to the Azerbaijani economy. Azerbaijan is home to a vast variety of landscapes. Over half of Azerbaijan's land mass consists of mountain ridges, crests, yailas, and plateaus which rise up to hypsometric levels of 400–1000 meters. The formation of climate in Azerbaijan is influenced particularly by cold arctic air masses of Scandinavian anticyclone, temperate of Siberian anticyclone, and Central Asian anticyclone.