The architectural tradition of Bulgaria is formed on ancient Thracian, Greek, and Roman architecture of which examples survive in several parts of the country. Three periods stand out in the development of distinct architectural styles over the ages. The first period was the Middle Ages, when Bulgarian and other architects constructed some of the great examples of early Byzantine architecture in territories that constituted the First Bulgarian Kingdom (see ch. 2). Many of these monuments are no longer within the boundaries of Bulgaria--notably the churches and monastery in the Lake Ohrid region of Yugoslavia--and others were destroyed during the centuries of Turkish rule. Among those that have survived within the confines of the country are some of the best examples of artistic expression and technology of the Byzantine period. These are a source of great pride for Bulgarians, who consider them part of their contribution to world culture. The next period of outstanding architectural development was the National Revival period of the late eighteenth century and the nineteenth century. Flourishing commerce gave new life to such towns as Plovdiv and Turnovo and created new urban centers in which affluent merchants and artisans built homes and public buildings in a richly ornamented style that came to be known as the National Revival style. These two-story structures made extensive use of stone and wood, the latter usually elaborately carved. The interiors were light and spacious. In addition to carved doors, ceilings, and built-in sideboards, the interiors were often decorated with wall paintings. Typical of National Revival architecture is the Rila Monastery; its interior walls are covered with frescoes, and its interior and exterior abound in carved wooden structural members and decorative details. The monastery, like other National Revival structures, reflects the Byzantine influence in the many large arched windows, arched ceilings, and arcades. The third period of distinguished architectural development is the contemporary one. Industrial growth since World War II and a rapidly growing tourist industry since the mid-1950s have called for large-scale construction of needed facilities. New resort towns arose on the Black Sea; industrial new towns grew in other locations; and hotels, apartment complexes, and public buildings were needed throughout the country. Although much of the architecture imitates the colossal style of Stalinist work, some of it is of high artistic quality and imagination. The Balkantourist Hotel in Turnovo and several hotels in Black Sea resorts are often singled out as outstanding examples of modern architecture; they combine traditional features with modern materials and techniques and blend them into a design that fits into the natural surroundings.