castra

Castra, castrorum, n., second declension, plural in form and singular in sense: castra sunt means "the camp is", one camp, not several. To count them Latin uses the distributive numerals, bina castra for two camps. The singular castrum does exist but means something else, a fort or fortified post, and in medieval Latin a castle.

Two idioms carry most of the traffic in military prose: castra ponere, "pitch camp", and castra movere, "strike camp and march". The word left a heavy mark on the map of Britain, where every Chester, Manchester, Lancaster and Doncaster marks a place the Romans garrisoned.