Found an excerpt from erenow.com about Järva County, Latin Jerwia:
"Jerwia (Est. Järva) was a province of medieval Livonia, conquered during the early 13th century in the course of the Baltic Crusades. It was surrounded by marshes, bordering Vironia to the east and Harria to the west, and inhabited by Estonians.
In 1217, the chieftains of Jerwia made peace with the German crusaders at Riga and accepted baptism. Nevertheless, they fought alongside other Estonian tribes against the crusaders in the battle of Fellin the same year. In 1219 the Order of the Sword Brethren subjected the land again, but the territory remained in dispute between the Danes and Riga until its status was settled by the Treaty of Stensby (1238). The king of Denmark gave Jerwia to the Teutonic Order on condition that the order would aid the king in the future and would not build fortifications there without the king’s permission. Jer- wia was the first district in Livonia that the order had not received from the bishops of Riga. The castle of Weissenstein (mod. Paide, Estonia), built in the 1260s, remained the northernmost stronghold of the order until the purchase of Harria and Vironia in 1346, and the bailiff of Jerwia who resided in Weissenstein rose into the circle of the most important officials of the order. By the end of the thirteenth century a town had grown up in Weissenstein, which was given the right to use the law code of the town of Riga in 1291.
The manorial system in Jerwia was complex. The Sword Brethren enfeoffed some land to vassals from the island of Gotland, while the Cistercian monastery of Roma on Gotland owned some villages; Gotlandic influences can be seen in the early church architecture in Jerwia. In the later centuries, however, the greater part of the land was retained by the order and not enfeoffed. A large number of free peasants and relatively better-off peasant households were characteristic of medieval Jerwia."