An easy explanation: 1396 to 1878. That and the slaughtering of any royal figures/aristocracy in BG during that period of time. Peasants do build castles, but only when are ordered by the aristocracy (where same aristocracy also has the money to pay for said castles). Unlike Arabs who left some interesting architecture behind (example: Spain), Turks did not seem to have been interested in leaving anything on BG territory, aside from reducing the population in half via quite violent means. The times of medieval castles and trubadurs in Western Europe, were times of trying to stay alive and keep one's identity and faith in BG. BG has some modest palaces (not castles) however: The Royal Palace in Sofia, Tsarska Bistritsa in Rila, Vrana Palace outside of Sofia, Banya Palace in Banya, Euxinograd Palace near Varna, Balchik Palace in Balchik. And BG has some more older fortresses: Kaliakra, Asen's Fortress, Tsarevets, Baba Vida, Belogradchik, Cherven, Pliska, Preslav, Tsepina, Ustra, Lyutitsa. Not magnificant like castles in rest of Europe, but still something that reminds of a different past....