The first cities in today's Poland, Kalisz and Elblag on the Amber Route to the Baltic Sea, were mentioned by Roman writers in the first century AD, but the first Polish settlement in Biskupin dates even further back to the 7th century BC.
Poland was first united as a country in the first half of the 10th century, and officially adopted Catholicism in 966 AD. The first capital was in the city of Gniezno , but a century later the capital was moved to Krakow, where it remained for half a millenium.
Poland experienced its golden age from 14th till 16th century, under the reign of king Casimir the Great, and the Jagiellonian dynasty, whose rule extended from the Baltic to the Black and Adriatic seas. In the 16th century, the Polish-Lithuanian Commonwealth was the largest country in Europe; the country attracted significant numbers of foreign migrants, including Germans, Jews, Armenians and the Dutch, thanks to the freedom of confession guaranteed by the state and the atmosphere of religious tolerance (rather exceptional in Europe at the time of the Holy Inquisition).
Under the rule of the Vasa dynasty, the capital was moved to Warsaw in 1596. During the 17th and the 18th centuries, the nobility increasingly asserted its independence of the monarchy; combined with several exhausting wars, this greatly weakened the Commonwealth. Responding to the need for reform, Poland was the 1st country in Europe (and the 2nd in the world, after the US) to pass a constitution. The constitution of May 3rd, 1791 was the key reform among many progressive but belated attempts to strengthen the country during the second half of the 18th century.