There are two principal sources of information on Roman camps.
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.
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.
Each 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 Principalis (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).
The 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 Decumana). 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 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 (the 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.
Located at the rear of the camp, this area generally consisted of cohortal barracks, granaries and stores.