The presentation will concern the stronghold on Ostrów Tumski in Poznań, which played an important role in the formation of the Piasts' rule. This was determined both by its favourable location and the functions associated with the site. An intensive expansion of the fortifications of the stronghold is recorded from around the middle of the 10th century, while the increase in its importance came during the reign of Mieszko I. At that time, it was the best fortified stronghold with a multi-part division. It is one of the places associated with conversion to Christianity and an episcopal missionary centre. The monumental architecture, the presence of a goldsmith's workshop and the attested processing of other metals, as well as numerous imports, attest to its importance. After a crisis in the 30s of the 11th century, the Poznan stronghold continued to be an important centre until the founding of the city on the left bank of the Warta in the middle of the 13th century.