As the Ottoman Empire continued their rule in the Balkans (Rumelia), Bosnia was somewhat relieved of the pressures of being a frontier province, and experienced a period of general welfare. A number of cities, such as Sarajevo and Mostar, were established and grew into regional centers of trade and urban culture and were then visited by Ottoman traveler Evliya Çelebi in 1648. Within these cities, various Ottoman Sultans financed the construction of many works of Bosnian architecture such as the country's first library in Sarajevo, madrassas, a school of Sufi philosophy, and a clock tower (''Sahat Kula''), bridges such as the Stari Most, the Emperor's Mosque and the Gazi Husrev-beg Mosque.