The kingdom was situated in the fertile alluvial plain, watered by the rivers Haliacmon and Axius, called Lower Macedonia, north of Mount Olympus. Around the time of Alexander I, the Argead Macedonians started to expand into Upper Macedonia, lands inhabited by independent Greek tribes like the Lyncestae and the Elimiotae, and to the west, beyond the Axius river, into the Emathia, Eordaia, Bottiaea, Mygdonia, Crestonia and Almopia; regions settled by, among others, many Thracian tribes. To the north of Macedonia lay various non-Greek peoples such as the Paeonians due north, the Thracians to the northeast, and the Illyrians, with whom the Macedonians were frequently in conflict, to the northwest. To the south lay Thessaly, with whose inhabitants the Macedonians had much in common, both culturally and politically, while to the west lay Epirus, with whom the Macedonians had a peaceful relationship and in the 4th century BC formed an alliance against Illyrian raids. Prior to the 4th century BC, the kingdom covered a region approximately corresponding to the western and central parts of the region of Macedonia in modern Greece.