Archaeologists have devised various methods to estimate the number of past site occupants. These techniques fall into a number of categories, but I will focus on two common approaches based on (1) the amount of roofed space each individual needs, and (2) the total site area and the number of persons per hectare. This report suggests that neither of these methods is fully appropriate for ancient military installations. Population estimates derived by the traditional techniques for the Byzantine fortress at Isthmia, Greece, are compared with numbers obtained by factors specific to a military camp. Through the examination of historical sources and contemporary military regulations, this paper presents a more appropriate coefficient for ancient fortresses, which had much denser populations than habitation sites. The use of such analogies, which are specific to the problem at hand, greatly increase the reliability of inferences.