guns were invented at that time, albiet a little later. they were strong enough to pierce a knights armor, making that other techology obsolete.
for the record, i dont think guns should be used by anyone who isnt wearing an military uniform.
i would think that the guns would share similar properties with crossbows. they have a fixed attack power.
and how do guns make it less of a strategy game? take FFTA, as they have guns and gunners. (albeit theyre horribly weak)