Well MED (http://quod.lib.umich.edu/cgi/m/mec/med ... s=23486012
) shows two distinct meanings for Captain, neither are the military rank which we now understand of course:
1. (a) The leader of an army, a general; the leader of a rebellion; also fig.; (b) ruler, leader, master.
2.(a) A military officer, one in command of a body of troops; chef ~, principal ~, the leader of an army; peti ~, a minor officer, one in charge of a small body or detachment of troops; also fig.; (b) the military governor of a castle or town; also fig.; (c) the commander of troops fighting aboard ships; chef ~, commander of the soldiers in a fleet; under ~, commander of the soldiers in one ship.
It seems to me that the question points towards the latter usage. The link should drop you into some quotes to illustrate the usage, although I can't specifically see archers there it does appear to match the idea of 'anyone in charge' of a place or body of men.