Gildas' principal work, De Excidio Britanniae, is a sermon in three parts condemning the acts of his contemporaries, both secular and religious. The only substantial source for post-Roman British history, it is of great value to the study of this period of history. The first part consists of Gildas' explanation for his work and a brief narrative of Roman Britain from its conquest under the principate to Gildas' time. He describes the doings of the Romans and the Groans of the Britons, in which the Britons make one last request for military aid from the departed Roman military. He excoriates his fellow Britons for their sins, while at the same time lauding heroes such as Aurelius Ambrosius, whom he is the first to describe as a leader of the resistance to the Saxons. He mentions the victory at the Battle of Mons Badonicus, in later texts attributed to King Arthur, though he is unclear as to the identity of the leader. Part two consists of a condemnation of five British kings, Constantine, Aurelius Conanus, Vortiporius, Cuneglas, and Maelgwn. As it is the only contemporary information about them, it is of particular interest to scholars of British history. Part three is a similar attack on the clergy of the time.