The sentry box; in military architecture; it consists of a small tower with loopholes or arrow slits; generally raised at the most salient angle of a fortress bulwark; that serves as shelter and protection for the sentries that protect the enclosure. there were sentry boxes in the oldest european fortifications of the middle ages. it is to believe that the first sentry boxes were of wood; like the matacanes; and that they were placed in times of war. nowadays the same term is used for international border crossing entrances; like the sentry box of san ysidro between tijuana; mexico and san diego; gallery