The construction of the castle began in the 10th century at the place of the Ancient Roman castell Bononia. The building of Baba Vida is tied to a legend, according to which a Danubian Bulgarian king who ruled at Vidin had three daughters: Vida, Kula and Gamza. Prior to his death, he divided his realm among the three. Vida, the eldest, was given Vidin and the lands north to the Carpathians, Kula was awarded Zaječar and the Timok Valley, and Gamza was to rule the lands west up to the Morava. Although Gamza and Kula married to drunkard and warlike nobles, Vida remained unmarried and built the castle in her city. The name of the castle means "Granny Vida".
Due to wars and castles being torn down as to prevent rebellions or just for the materials during ottoman times Baba Vida remains the only fully intact castle in Bulgaria.
Here’s also a pretty painting of medieval Vidin
Bdin, by Vasil Goranov