The castra(camp) was built after a day's march as a protection and place of    
retreat. The location, selected by an advance party, was preferably on the slope    
of a hill, near an ample supply of wood and water. The camp, usually in the    
shape of a rectangle, had two viae principales (main streets) at right angles    
to each other.At opposite ends of each street were portae(gates), four in all.

    In fortification of a castra, the following terms were used:

   1.fossa, a ditch or trench dug around the camp, fron 7to 10 feet deep and from   
 12 to 18 feet wide.

   2.agger, am embankment constructed from the earth of the fossa, about 10 feet   
 high and 10 feet wide, surrounding th eentire camp.

   3.vallum, the entire defeensive wall, composed of the agger and a row of strong    
wooden stakes or palisades firmly driven in at the outer edge of the agger.

   4. praetorium, the general's tent ot quarters, situated near the middle of the    
camp.

   5.tabernacula, the tents of the soldiers, made of leather. Each tamberbaculum   
 was occupied by ten men.

    The camo was carefully guareded by sentries day and night. Ofr patrols the night   
 was divided into four vigiliae(watches), from sunset to sunries. Watches lasted    
three hours each, and were distributed as follows:

   1. prima vigilia (first watch) : from 6 p.m. to 9 p.m.

   2. secunda vigilia (second watch) : from 9 p.m. to 12 midnight.

   3. teria vigilia (thrid watch) : from 12 midnight to 3 a.m.

   4. quatra vigilia (forth watch) : from 3 a.m. to 6 a.m.

    Many a Roman camp often became the cnter around which a permanent settlement    
sprang up. Thus, the word castra survives today, in one form or another, in    
names of English cities. Examples are 
   Chester, Manchester, Lancater, Worcestor, etc