In the Roman Empire, a castra (the plural form of castrum, castri, a fortification) was a Roman military camp. Roman camps were always constructed according to a certain pattern, with two main crossing streets: the "Cardus Maximus", running north and south, and the "Decumanus Maximus" running east and west, dividing the camp into four quarters. These streets terminated at four gates. The forum was located at the intersection of the Cardus Maximus and the Decumanus Maximus.

The rest of the streets and constructions were parallel to these streets, which formed a quadricular pattern widely used in cities.

Many villages in Europe originated as Roman military camps and still show traces of their original pattern (e.g. Castres in France, Barcelona in Spain). The pattern was also used by Spanish colonizers in America following strict rules by the Spanish monarchy for founding new cities in the New World.

Many of the towns of England still retain forms of the word castra in their names -- Lancaster, Chester and Manchester, for example.