would not invade her country again. Main trading artery For hundreds of years the river was one of the main trading arteries of Poland, and the castles that line its banks were highly prized possessions. Salt, timber, grain, and building stone were among goods shipped via that route between the 10th and 13th centuries. In the 14th century the lower Vistula was controlled by the Teutonic Knights Order, invited in 1226 by Konrad I of Masovia to help him fight the pagan Prussians on the border of his lands. In 1308 the Teutonic Knights captured the Gdańsk castle and murdered the population. Since then the event is known as the Gdańsk slaughter. The Order had inherited Gniew from Sambor II, thus gaining a foothold on the left bank of the Vistula. Many granaries and storehouses, built in the 14th century, line the banks of the Vistula. In the 15th century the city of Gdańsk gained great importance in the Baltic area as a centre of merchants and trade and as a port city. At this time the surrounding lands were inhabited by Pomeranians, but Gdańsk soon became a starting point for German settlement of the largely fallow Vistulan country. Before its peak in 1618, trade increased by a factor of 20 from 1491. This factor is evident when looking at the tonnage of grain traded on the river in the key years of: 1491: 14,000; 1537: 23,000; 1563: 150,000; 1618: 310,000. In the 16th century most of the grain exported was leaving Poland through Gdańsk, which because of its location at the end of the Vistula and its tributary waterway and of its Baltic seaport trade role became the wealthiest, most highly developed, and by far the largest centre of crafts and manufacturing, and the most autonomous of the Polish cities. Other towns were negatively affected by Gdańsk's near-monopoly in foreign trade. During the reign of Stephen Báthory Poland ruled two main Baltic Sea ports: Gdańsk controlling the Vistula river trade and Riga controlling the Western Dvina trade. Both cities were among the largest in the country. Around 70% the exports from Gdańsk were of grain. Grain was also the largest export commodity of the Polish–Lithuanian Commonwealth. The volume of traded grain can be considered a good and well-measured proxy for the economic growth of the Commonwealth. The owner of a folwark usually signed a contract with the merchants of