it held. A charter from the 1130s mentioned 30 administrative posts in Novgorod territory, where revenues were collected regularly and sent as a tithe to the Novgorod bishop. Throughout the 12th century, Novgorod utilized the Baltic-Volga-Caspian trade route, not only for trading but also for bringing food from the fertile Oka region to their city. Rostov-Suzdal comprised the territory of the important Oka region and lands along the vital Sheksna River. This river lay in the Northern Volga tributary region. Whoever controlled the river was able to block food supplies causing a famine in Novgorod. Perhaps due to these fears, Novgorod led a failed invasion of Suzdal in 1134. They tried again and succeeded in 1149. Alternatively, Novgorod, in a bid to appease Suzdal, accepted some Suzdalians as rulers of Novgorod. Despite these events, Suzdal still blocked off trade to Novgorod twice and intercepted Novgorod's tributes. In 1136, the Novgorodians dismissed Prince Vsevolod Mstislavich and over the next century and a half were able to invite in and dismiss a number of princes. However, these invitations or dismissals were often based on who was the dominant prince in Rus' at the time, and not on any independent thinking on the part of Novgorod. The city of Pskov, initially part of Novgorod Land, had de facto independence from as early as the 13th century after opening a trading post for merchants of the Hanseatic League. Several princes such as Vsevolod Mstislavich (before 1117–1138) and Dovmont (c. 1240–1299) reigned in Pskov without any deference to, or consultation with, the prince or other officials in Novgorod. The independence of the Pskov Republic was acknowledged in the 1348 Treaty of Bolotovo. However, the Archbishop of Novgorod continued to head the church in Pskov and kept the title "Archbishop of Novgorod the Great and Pskov" until 1589. The Republic was the subject of political rivalry between Poland and Lithuania on one side and the Grand Duchy of Moscow on the other side in the 14th and 15th centuries. In the 12th–15th centuries, the Novgorodian Republic expanded east and northeast. The Novgorodians explored the areas around Lake Onega, along the Northern Dvina, and coastlines of the White Sea. At the beginning of the 14th century, the Novgorodians explored the Arctic Ocean, the Barents Sea, the Kara Sea, and the West-Siberian river Ob. The lands to the north of the city, rich with fur, sea fauna and