 Complete the following phrase:  Gardoš Fortress, also known as the Zemun Fortress or the Citadela, is the oldest preserved part of Old Zemun and is considered one of Belgrades best scenic viewpoints. The existing walls are originating from the 14th and the 15th century. The fortress was built as part of the Hungarian project of restoring or building fortresses on their southern borders, including today also ruined but surviving Kupinik and Slankamen, as a response to the incoming Ottoman invasion. Gardoš had a major role in 