He established several inland centres of operation. One of his cardinal forts was on Lough Ree, north of Athlone. Another fort was at a point called Lyndwachill on Lough Neagh, while others were on the high ground southwest of Lough Lene and on the major island of this lake, which still bears his name and was where he dominated the Leinster midland from. In 843, King Niall Caille met Turgesius in battle when defending his ancestral province of Ulster. Niall fought against both the Vikings of Lough Neagh and a further party who sailed into Lough Swilly. On the plain of Moynith, the forces of King Niall and Turgesius met and, according to the annals, "a countless number fell". The result of the battle was that King Niall had vanquished Turgesius. In the year 843 or 844 Turgesius was killed by Máel Sechnaill I (also called Malachy), King of Meath, and a subordinate of Turgesius. There are conflicting reports of how Turgesius met his death. He is reputed to have been put to death by drowning in Lough Owel near Mullingar. Local tradition says that Máel Sechnaill governed under Turgesius and is believed to have asked advice from Turgesius how best to rid the area of a recently invading flock of birds who were causing damage. Without a second thought, Turgesius recommended destroying their nests and this inspired Maél Sechnaill to do the same in order to rid his territory of the Vikings. Another story of his death states that he demanded Melaghlin's daughter's hand in marriage. While pretending to agree, Melaghlin sent Turgesius 12 beardless youths, disguised as his daughter and her attendants, who were in reality assassins. The seventeenth century historian Geoffrey Keating wrote in "Foras Feasa ar Éirinn"  By the twelfth century, when "The War of the Irish with the Foreigners" ("Cogad Gaedel re Gaillaib") was composed to magnify the achievements of Brian Bóruma, Turgesius had become a major figure. Gerald of Wales, who may have had access to a version of this work, included similar accounts in his "Topographia Hibernica" although these accounts are now not always deemed trustworthy. According to "The War of the Irish with the Foreigners", Turgesius was married to Ottar or Ota (commonly thought to be Old Norse "Auðr", "Odda" or another name beginning in Odd-), who took possession of the cathedral at Clonmacnoise and gave audiences seated on the great altar.