Think of these settlements as a “manor” or a “land”, ruled by a landlord. Considering the time as the late 12th to early 13th Century, the average model of guards would be vassals that, by pledging allegiance to a liege (landlord), subjected to his dominium as knights to protect his castle and lands in exchange for better living conditions, maybe for the education of their children or just as a “work” but build outside of monetary values. Remember, the only places that escape this model in its entirety in Europe are: Italy, most of Spain and some regions on the East. I hope I could answer your question and, if there are any doubts, ask me again as a reply.