Poland's Bronze- and Iron-Age cultures are known mainly from archeological research. Poland's Early Bronze Age cultures began around 2300-2400 BC, whereas the Iron Age commenced c. 700-750 BC. By the beginning of the Common Era, the Iron Age archeological cultures described in the main article no longer existed. Given the absence of written records, the ethnicities and linguistic affiliations of the groups living in Central and Eastern Europe at that time are speculative; there is considerable disagreement about their identities. In Poland, the Lusatian culture, which spanned the Bronze and Iron Ages, became particularly prominent. The most famous archeological discovery from that period is the Biskupin fortified settlement ("gród") that represented early-Iron-Age Lusatian culture.