They're described as "German", and are fighting "Prussia". That gives seems to give some hard dates to start with; from 1525 (the formation of the state Prussia) - 1871 (when Prussia absorbs Germany).
But maybe it's earlier, "Prussia" could just be the name of the region. Certainly not later. Though this setting pretty clearly isn't 19th-cetury to start with; ideally we'd be more towards the early end, or as said even earlier; firearms not being prevalent (or even extant?) in this story.
As for Germany--what "Germany"? Not the German Confederation, that being both too late (starting 1815) and far too large (whereas this story has a single territory ruled by a small group of allied nobles). Notably, we haven't had anyone called a King or any other head-of-state-y title, either (even though the buck seems to stop at the head family). So, rather, theirs is probably a small, mostly-independent German state of the relatively loose association of states that formed the Holy Roman Empire? (Vs. either a Prussian state or just folks in the Prussian region.)
The existence of "Prussia" and "Poland" as even place-names would seem to put a practical lower bound on any dates, maybe something like the ninth century, give-or-take? (Which also implies, not before the Holy Roman Empire, certainly)