Poland in the Early Middle Ages

The most important phenomenon that took place within the lands of Poland in the Early Middle Ages, as well as other parts of central and eastern Europe, was the arrival and permanent settlement of Slavic peoples. The Slavic migrations in the area of contemporary Poland started in the second half of the 5th century CE, about a half century after these territories were vacated by Germanic tribes. The first waves of the incoming Slavs settled the vicinity of the upper Vistula River and elsewhere in the lands of present southeastern Poland and southern Masovia. Coming from the east, from the upper and middle regions of the Dnieper River, the immigrants would have had come primarily from the western branch of the early Slavs known as Sclaveni, and since their arrival are classified as West Slavs. Their early archeological traces belong to the Prague-Korchak culture, which is similar to the earlier Kiev culture.

From there the new population dispersed north and west over the course of the 6th century. The Slavs lived from cultivation of crops and were generally farmers, but also engaged in hunting and gathering. The migrations took place when the destabilizing invasions of eastern and central Europe by waves of people and armies from the east, such as the Huns, Avars and Magyars, were occurring. This westward movement of Slavic people was facilitated in part by the previous emigration of Germanic peoples toward the safer and more developed areas of western and southern Europe. The immigrating Slavs formed various small tribal organizations beginning in the 8th century, some of which coalesced later into larger, state-like ones. Beginning in the 7th century, these tribal units built many fortified structures with earth and wood walls and embankments, called gords. Some of them were developed and inhabited, others had a very large empty area inside the walls.

...
Wikipedia