I have just recently discovered that Lithuania and the baltic lands more in general were the last european lands to be Christianized, and I kinda... can't wrap my head around it. I can't form any kind of image in my mind when I hear "tribal pagans in 13th century Europe". So I ask: what do we know about those pagan tribes that were able to defeat a 13th century christian feudal army at the Battle of Saule? I am mainly interested in their political organization, religion and culture.
Thing is, we don't know much since neither of those tribea left written sources for us. You can try looking into Livonian Rhymed Chronicle, which gives insights to their political and military decisions from the period. Archaeological data is abundant, but most reports are in Lithuanian only. Basically it seems that both Samogitians and Semigallians were living in unions of smaller sub-tribes, choosing a single leader for military campaigns from amongst higher local nobility. Religious sources are even scarcer, but it's safe to say that the absolute majority within the region followed their version of Baltic paganism, albeit probably with minor variations depending on the tribe. I hope this was helpful at all.