this theory, however the exact location of the early center of the state is disputed. According to Gyula Kristó the center was located between the Danube and Tisza rivers, but the archaeological findings imply a location in the region of the Upper Tisza. Constantine VII's De Administrando Imperio, written around 950 AD, tries to define precisely the whole land of the Hungarians, or Tourkia. Constantine described the previous inhabitants of Hungary (e.g., the Moravians), described early Hungarian settlements and neighbors, and located Hungarian rivers (Temes, Maros, Körös, Tisza, Tutisz). Constantine had much more knowledge about the eastern parts of Hungary; therefore, according to one theory, Tourkia did not mean the land of the whole federation, but a tribal settlement and the source of the description of Hungary could have been Gyula whose tribe populated the five rivers around 950. According to another hypothesis, mainly based on Constantine's description, the Hungarians started to really settle western Hungary (Transdanubia) only after 950, because the eastern part of the country was more suitable for a nomadic lifestyle. Due to changed economic circumstances, insufficient pasturage to support a nomadic society and the impossibility of moving on, the semi-nomadic Hungarian lifestyle began to change and the Magyars adopted a settled life and turned to agriculture, though the start of this change can be dated to the 8th century. The society became more homogeneous: the local Slavic and other populations merged with the Hungarians. The Hungarian tribal leaders and their clans established fortified centers in the country and later their castles became centers of the counties. The whole system of Hungarian villages developed in the 10th century. Fajsz and Taksony, the Grand Princes of the Hungarians, began to reform the power structure. They invited Christian missionaries for the first time and built forts. Taksony abolished the old center of the Hungarian principality (possibly at Upper Tisza) and sought new ones at Székesfehérvár and Esztergom. Taksony also reintroduced the old style military service, changed the weaponry of the army, and implemented large-scale organized resettlements of the Hungarian population. The consolidation of the Hungarian state began during the reign of Géza. After the battle of Arcadiopolis, the Byzantine Empire was the main enemy of the Hungarians. The Byzantine expansion threatened the Hungarians, since the subjugated First Bulgarian Empire was allied with the Magyars at that time. The situation became more difficult for the principality when the Byzantine Empire