Until the end of the 19th century house-builders and stair carpenters occasionally included a passive "alarm" system in the homes they built—a "trip-step" rising a few inches higher than the other steps—that would cause an unwary and unknowing intruder to stumble in the night and awaken slumbering occupants. The idea was adapted from an earlier military defense strategy used by medieval stone masons who constructed uneven steps in castle stairwells hoping to thwart invading foreign armies.