from the south led to the settlement also in mountain areas. In the 8th century, the Slavs increased their agricultural productivity (usage of iron plow) along with further development of crafts. Higher productivity initiated changes in the Slavic society, released a part of human resources previously required for farming and allowed to form groups of professional warriors. The Slavs began to build heavily fortified settlements (hradisko - large grad) protected by strong walls (8–10 m) and trenches (width 4–7 m, depth 2–3.5 m). Among the oldest belong Pobedim, Nitra-Martinský Vrch, Majcichov, Spišské Tomášovce and Divinka. The neighborhood with Avars raised unification process and probably also formation of local military alliances. The archaeological findings from this period (such as an exquisite noble tomb in Blatnica) support the formation of a Slavic upper class on the territory that later became the nucleus of Great Moravia. A series of Frankish-Avar wars (788-803) led to the political fall of the khaganate. In 805, the Slavs attacked again. Their offensive aimed mainly on the centers of Avar power - Devín and Komárno. The Avars were not able to resist attack and they were expelled to the right bank of Danube. The Slavs from Slovakia probably participated also in further conflicts between small Slavic dukes and remaining Avar tarkhans. Principality of Nitra The Conversio Bagoariorum et Carantanorum, written around 870, narrates that Moimir, the leader of the Moravians, expelled one Pribina, forcing him to cross (or come up) the Danube and join Radbod, who was the head of the March of Pannonia in the Carolingian Empire from around 830. Radbod presented Pribina to King Louis the German who ordered that Pribina be instructed in the Christian faith and baptised. Three of the eleven extant copies of the Conversio also contain an out of context statement which says that Adalram, who was Archbishop of Salzburg between 821 and 836, had once consecrated a church on Pribina's "estate at a place over the Danube called Nitrava". According to a widely accepted scholarly theory, "Nitrava" was identical with Nitra in present-day Slovakia and the forced unification of Pribina's Principality of Nitra with Mojmir's Moravia gave rise to the development a new state "Great Moravia". Between 800 and 832, a group of Slavic forthills in Slovakia quickly arose and disappeared. Archaeological research confirmed the fall of several important central forthills approximately around the time when Pribina was expelled, e.g. Pobedim