Casting a vote for late medieval; something comparable to the 15th-16th centuries. At this point, gunpowder weapons like culverins, bombards and arquebuses were becoming more and more commonplace, but still shared the battlefield with more traditional compositions of crossbows, armored cavalry and infantry. I think this would allow for diversity in faction militaries, and people could always elect to downplay, or completely ignore modern trends and play something like the Aztecs in the face of another player's Spain-analogous nation.