This is a great question!
During the late Medieval period, the Baltic tribes were nearly constantly at war, whether or land or religion. One of the major forces during the period you mention was Denmark, which, under King Valdemar II conquered much of Estonia.
During this time, there were several wars to expand Christianity further north, although several pagan tribes, including the Galindians, maintained some degree of cultural autonomy for a time.
From what I can gather, the militaries in Denmark, Lithuania, etc., mirrored the armies present in other feudal European societies. What seems to have distinguished the Lithuanian forces was its mobility, since it had several enemies on various fronts. This meant that they heavily relied on horses, and had several tactics, such as spear-throwing, that made use of these animals. They also prioritized those soldiers with the most experience, putting them at the front of the ranks, and the least experienced at the back.
