The Great Basilica “St. Sofia" in the city of Pliska, capital of the First Bulgarian Empire between 680 and 893 AD, was built between 866 and 875 AD, after Bulgaria’s adoption of Christianity as the official state religions in 865 AD under Knyaz Boris I Mihail (r. 852-889; 893 AD).
The Great Basilica in Pliska, the first capital of Bulgaria south of the Danube, was the largest Christian cathedral in Europe in the Middle Ages. It was long 102.5 meters and wide 30 meters, which means it was 20 meters longer than the Hagia Sophia Cathedral in Constantinople, the titular temple of the Ecumenical Patriarchate during the period of the Byzantine Empire, and about 30 meters longer than the San Pietro in Vincoli Cathedral in Rome. Thus, the Great Basilica in Bulgaria’s Pliska was technically the largest Christian temple in Europe until the 17th century, i.e. until the completion of the Papal Basilica of St. Peter in the Vatican in 1629 AD.
The Great Basilica in Pliska was built with huge white limestone quadrae from the quarries in the nearby town of Kyulevcha. Around the basilica there was a large monastery complex and the residence of the Bulgarian Archbishop (between 870 and 917 AD), and the Bulgarian Patriarch (from 917 AD onwards). In this monastery complex, Bulgarian archaeologists have found a scriptorium for the “production" of medieval books in Old Bulgarian, also known as Church Slavonic. As Bulgaria’s National Museum of History points out, “it is with these books that the monastery monks and missionaries converted [to Christianity] the peoples of modern-day countries Russia, Ukraine, Belarus, Romania, Moldova, Serbia in the 9th-10th century."
The Great Basilica in Pliska was still standing until the Late Middle Ages but was razed to the ground by the Ottoman Turks after their invasion at the end of the 14th century, and in the 15th century, because according to the laws of the Islamic Ottoman Empire no Christian temple could stand taller than a Muslim man mounted on a horse. The construction material from the unique buildings in Pliska was used by the Ottomans for the construction of the Turkish military barracks and the Tombul Mosque in the nearby city of Shumen,, and whatever had been left of it by the 19th centur – for the construction of the Varna-Ruse railway in 1866 under Ottoman governor Midhat Pasha.