To make up for losses from the plague and to replace the partially exterminated native population, the Order encouraged immigration from the Holy Roman Empire (mostly Germans, Flemish, and Dutch) and from Masovia (Poles), the later Masurians. These included nobles, burghers, and peasants, and the surviving Old Prussians were gradually assimilated through Germanization. The settlers founded numerous towns and cities on former Prussian settlements. The Order itself built a number of castles ("Ordensburgen") from which it could defeat uprisings of Old Prussians, as well as continue its attacks on the Grand Duchy of Lithuania and the Kingdom of Poland, with which the Order was often at war during the 14th and 15th centuries. Major towns founded by the Order included Allenstein (Olsztyn), Elbing (Elbląg), Klaipėda (Memel), and Königsberg, founded in 1255 in honor of King Otakar II of Bohemia on the site of a destroyed Prussian settlement.