At the height of its power in the 15th century, the Grand Duchy of Lithuania ruled over the territory of present-day Belarus, Latvia, Lithuania, and Ukraine, and parts of Estonia, Moldova, Poland, and Russia. In the Union of Lublin of 1569, the Grand Duchy and the Kingdom of Poland merged to form the Polish-Lithuanian Commonwealth. The commonwealth had a highly developed legal and administrative system, based on local land courts that decided civil cases involving the gentry and castle courts that dealt with other local matters, including criminal cases. Courts were required to maintain detailed records of their proceedings, which were kept in old Byelorussian and in Polish. In the 19th century, the court records of the Grand Duchy were centralized in Vilnius, and eventually became part of the manuscript collections of the Vilnius University Library. The library now holds 543 books of court acts of the Grand Duchy of Lithuania dating from 1540 to 1845. These books are a unique source for the histories of Lithuania, Poland, and Belarus. Shown in this volume are the Ukmerge castle court year book for 1595 and some records for 1686–88. Ukmerge is 78 kilometers from Vilnius, in east-central Lithuania. In 1655, during the Second Northern War of 1655–60, Swedish and Russian armies plundered the city.