From the time of its establishment the city of Dubrovnik was under the protection of the Byzantine Empire; after the Fourth Crusade the city came under the sovereignty of Venice 1205–1358 CE, and by the Treaty of Zadar in 1358, it became part of the Hungarian-Croatian Kingdom. Following the 1815 Congress of Vienna, the city was annexed by Austria and remained part of the Austro-Hungarian Empire until the conclusion of the First World War. From 1918 to 1939 Dubrovnik was part of the Zetska Banovina District that established its Croatian connections. From 1945 to 1990 Croatia would become part of the Socialist Federal Republic of Yugoslavia. One of the most striking features of the historic city of Dubrovnik, and that which gives its characteristic appearance are its intact medieval fortifications. Its historic city walls run uninterrupted encircling the Old-City. This complex structure of fortification is one of the most complete depictions of medieval construction in the Mediterranean, consisting of a series of forts, bastions, casemates, towers and detached forts. Within the Old City are many medieval churches, cathedrals, and palaces from the Baroque period, encircled by its fortified wall, which would ensure its listed place by UNESCO as a world heritage site in 1972. The Old Town is not only an architectural and urban ensemble of high quality, but it is also full of museums and libraries, such as the collection of the Ragusan masters in the Dominican Monastery, the Museum of the History of Dubrovnik, the Icon Museum, and the libraries of the Franciscan and Dominican Monasteries. It also houses the archives of Ragusa, which have been kept continuously since the 13th century and are a very important source for Mediterranean history. The archives hold materials created by the civil service in the Republic of Ragusa.