Peasants or the cheapest militia spear unit are typically my garrisons in BI. I also build a fort on the roads between each settlement and place one peasant unit in each. That's usually enough to reduce the brigands emerging as often. I haven't played BI that much though. I'd rather not use peasants at all, but for some factions their low upkeep is too crucial to ignore. In RTW I use town militia units or warbands for garrison duties, or in my current mod, garrison auxiliaries.
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