Aetolia, a region in west-central Greece roughly shaped like a triangle with its base on the Corinthian Gulf, its apex at Mt. Tymphrestus, and its sides along the lower and middle *Acheloüs river-valley on the west, and a series of mountains from Mt. Oxya to Mt. Gkiona on the east. The topography of the region is rugged, a factor that played a significant role in Aetolia's history, serving as a natural deterrent to invading armies, and contributing to the widespread practice of *brigandage. Along the region's southern coast there are few harbours, although the area was settled from early times. Towns like *Pleuron, Olenus, Pylene, Calydon, and Chalcis are all known to Homer (Il. 2. 638–44).Politically and economically Aetolia remained backward into the 5th cent. bce. *Thucydides (2) mentions settlements at Poti-dania, Crocylium, Tichium, Aegitium, and Proschium, but he describes them as small unfortified villages. During this period, Aetolia was organized as an ethnos (see ethnicity), consisting of at least three major territorial groups: the Ophioneis, the Apodoti, and the Eurytanes.