The region of northwestern Bulgaria is bordered by the Danube River, Romania to the north, and the Balkan Mountains and Serbia to the southwest. It includes many diverse landscapes between mountains covered in dense forests and the wide and wonderful Danube plains. It also includes the Vrčanski Balkan Nature Reserve, which is an important habitat for butterflies in Bulgaria, in addition to... Belogradchik Rocks, which are natural sandstone rock formations, and the limestone geology of this area has produced a karst topography with many stunning caves that are tourist attractions in themselves, the most famous of which is Magura Cave, which has been used by humans since at least the end of the Paleolithic Age. He is known for his prehistoric cave paintings. In addition, the Romans built fortifications and military borders along the southern banks of the Danube in the northwestern Bulgarian region, making it full of medieval castles, fortresses and monuments, the most famous of which is Baba Vida, one of Bulgaria's last strongholds during the Ottoman invasion.