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The primary purpose of a camp or fort was not to withstand attack but to provide a safe staging area for troops to organize before
deployment in battle. This is evident by towers being built-in or set flush with the outside camp walls. Only in the twilight years of the
empire, when its military might was waning and it was being besieged by its enemies, did towers begin to protrude from fort walls to
improve the field of fire.

The typical legionary camp adhered to a playing card shape with rounded corners. It was surrounded by a trench (
fossa) with a mound
ager) crowned with a wall (vallum). The vallum in a marching camp was a wood palisade but in permanent forts the defensive walls
were of heavy timber or stone.

In the field, a legion would march for five hours before it halted. Engineers preceded the main body and staked out a camp site. Upon
arrival, the legionnaires would excavate the
fossa, construct the vallum, and erect their tents before dinner. On the day of departure
breaking camp involved backfilling the
fossa to prevent occupation of the site by hostiles.

Camps, whether marching or permanent, were always constructed near a reliable source of water for drinking and bathing. The
engineers sited them on ground with sufficient slope to carry off waste water. Latrines in permanent forts were specifically designed so
that water flowed continuously through them to carry off disease-causing effluent.

A cleared space (the
intervallum) with a road (the via Sagularis, known as cloak street) paralleled the vallum around the interior. The
intervallum reduced vulnerability to incoming fire (viz. incendiaries) by distancing the tents from enemy projectiles. It also provided a
rallying/marshalling point for troops to defend the ramparts or march out on patrol.

Unlike modern armies, there was no central mess hall. It was the responsibility of each
contubernium (a section of 6-10 men) to
requisition food and cook their own meals (usually over open fires in marching camps). In permanent forts or fortresses communal
beehive-shaped ovens were often recessed into the
agger that bordered the intervallum.

castra was equipped with four gates defended by towers. The main gate (Porta Praetoria) was typically oriented to face the
direction of maximum danger. From this gate the
via Praetoria ran straight to a T-shaped junction where it intersected with the via
(which had gates at either end, the Porta Principalis Sinistra (left) and the Porta Principalis Dextra (right).

Roads were elevated in the centre so that surface water ran off into gutters flanking either side (and thence out of the camp).

via Praetoria stopped in front of the principia (legion headquarters). Directly behind the principia the road continued under the
name of the
via Decumana (derived from the location of the 10th maniple in a Republican camp) and ran to the rear gate (Porta
). Since camp supplies were received by way of the Porta Decumana, it was alternatively known as the Porta Quaestoria.
Parallel to the
via Principalis and directly behind the Principia was the via Quintana (5th street) which was on occasion equipped with
gates where it met the ramparts at either end.

Internal Camp Divisions

The via Principalis and the via Quintana split the camp into three sections. The physical location of the barracks, the principia and the
tribune’s houses was generally fixed in a camp.

The Praetentura:

The zone in front of the
principia housed cohortal barracks. Quarters for the senior officers (above the level of centurion but below that of
legate) ran along the
via Principalis. Cleanliness was important to Romans and every permanent camp had baths within its walls or in
close proximity outside. Baths usually had an exercise hall and a courtyard for sports (a
palaestra) not unlike a modern YMCA.

The Latera Praetorii:

This area was between the
via Principalis and the via Quintana. To one side of the principia was the commanding officer’s quarters
praetorium). In a permanent fort it was a separate building constructed in the fashion of a Mediterranean villa. The tribunal, altars,
auguries [
auguratorium], granaries [horraea], the armoury [armamentarium], workshops [fabrica], the hospital [valitudinarium], the
legionary cavalry barracks [
equites] and scouts [speculatores], were also located in the latera praetorii.

The Rententura

Located at the rear of the camp, this area generally consisted of cohortal barracks, granaries and stores.

There are two principal sources of information on Roman

  • Polybius (ca. 200 to 118 BC), an Achaean, who
    described contemporary camps in (chapters 19-42 of
    Book 6)  Histories. He was a tutor to the Scipio family
    and accompanied Scipio Aemilianus (a natural son of
    Aemilius Paullus) to the siege of Carthage in 146 BC.

  • Pseudo-Hyginus , an unknown author who composed
    De Munitionibus Castrorum, a work on military camps,
    sometime between the 2nd and 4th century.