the layout of the Zoroastrian temples of fire and the Kuban Bulgar centre, Humarin citadel, situated 11 km to the north of the town Karachayevsk, where the pottery belonged to the Saltovo-Mayaki culture. Kuznecov also found a connection in the plan of the Danube Bulgars sanctuaries at Pliska, Veliki Preslav, and Madara. The architectural similarities include two squares of ashlars inserted one into another, oriented towards the summer sunrise. One of these sites was transformed into a Christian church, which is taken as evidence that they served a religious function. The view of the Parthian and Sasanian influence, which Franz Altheim also argued, is considered debatable, showing the cultural impact of the Iranian world on communities in the Pontic–Caspian steppe. Many scholars believe that the square shape, with the north–south and east–west axis of the Bulgar sacral monuments is very similar to those of Turkic khagans in Mongolia. However, that the Bulgar residence in Pliska and Palace of Omurtag were inspired by the Byzantine architecture is considered indisputable. Christianity had already begun to penetrate, probably via their Slavic subjects, when it was adopted in the First Bulgarian Empire by Knyaz Boris I in 865 as a state religion. There was interest in Islam as well, seen in the book Answers to the Questions of the King of the Burgar addressed to him about Islam and Unity by the Abbasid caliph Al-Ma'mun (813–833) for the Pontic/Bosporan Bulgars, while it was officially adopted in Volga Bulgaria as a state religion in 922. Language The origin and language of the Bulgars has been the subject of debate since around the start of the 20th century. It is generally accepted that at least the Bulgar elite spoke a language that was a member of the Oghur branch of the Turkic language family, alongside the now extinct Khazar and the solitary survivor of these languages, Chuvash.Angelov 1971: §II.2 Some scholars suggest Hunnic had strong ties with Bulgar and to modern Chuvash and classify them as separate Hunno-Bulgar languages. According to P. Golden this association is apparent from the fragments of texts and isolated words and phrases preserved in inscriptions. In addition to language, their culture and state structure retain many Central Asian features. Military and hierarchical terms such as khan/qan, kanasubigi, qapağan, tarkan, bagatur and boila appear to be of Turkic origin. The Bulgar calendar within the Nominalia of the Bulgarian khans had a twelve-year