BELGRADE Although Belgrade, is a very old city, known as Singidunum in the Roman times, it hasn't got many monuments from the past. It has been the target of many invaders throughout its history, so that was destroyed and rebuilt many times. There are almost no structures dating back to the old Roman times or medieval monuments. There are some monuments dating back to the times under Turkish rule. The best preserved of the latter is Kalemegdan, the old core of the Belgrade fortress. Apart from some Roman foundations and walls dating back to Despot Stefan's period (early 15th century), almost all its other fortifications were built in the 18th century by the Austrians. The complex system of extensive heavy ramparts, towers, bridges, turrets, moats and passages encompasses a number of fine fortress gateways, a clock tower, an old Roman well, and the Ruzica Church in the upper part; and a baroque gateway and the medieval Nebojsa Tower in the lower part. The fortress has been turned into a city park offering a wide variety of monuments from more recent times. Particularly attractive is a sculpture by Ivan Mestrovic - Pobednik (The Victor). The Military Museum in Kalemegdan, has a large collection of weapons and documents illustrating the history of Belgrade and Yugoslavia from the earliest times to date. Landmarks in the vicinity od Kalemegdan are the Cathedral Church (Saborna crkva), from the first half of the l9th century, the Princess Ljubica mansion (Konak kneginje Ljubice) - the court of renewed Serbia's first Prince - built in 1831, the Bajrakli Mosque, from the 17th century, and the building of Serbia's first school of higher learning, which now houses the Dositej Obradovic and Vuk Karadzic Museum, dedicated to the two founders of modern Serbian culture. Noteworthy among Belgrade's museums is the National Museum, with its rich prehistoric and classical collections, medieval section and art gallery , the Ethnographic Museum, the Gallery of Frescoes and the Museum of the Serbian Orthodox Church.