The Roman Military in Britain
The Roman fortification, whether it was a temporary overnight camp in
enemy territory, an auxilliary outpost fort set to guard a strategic
location, or a large fortress to garrison the might of the Roman
legions, was almost invariably built to the same basic formula.
Although Roman camps have been found of various shapes, by far the most
common form is that of a quadrilateral with parallel sides and rounded
corners. The outline of the fortifications can generally be used as a
rough guide to the period in which they were built. The traditional
'playing-card' shape is typical of the first century, whereas by the
second and third centuries, squat, square-shaped forts predominate. By
the fourth century the Roman military began to depart from this strict,
angular form, as attested by the ovular outline of the Saxon Shore Fort
at Anderitum (Pevensey).
A Roman camp was always enclosed by a defensive system comprising at least three components;
- At least one ditch or fosse.
- An inner rampart or agger containing the ditch outcast.
- A palisade or vallum surmounting the rampart.
Within the defences, the camp was divided into three main areas:
- Praetorium - Situated at the geometric centre of the camp,
this area was where the commander's tent was pitched and the military
standards grounded. The areas to left and right of the praetorium
outwards up to the defences of the camp (marked D[extra] and S[inistra]
on the accompanying plan) were occupied by the various military stores,
hospitals, workshops and the like. In a temporary encampment in enemy
territory, this area would house the grain and military supplies along
with the carts and baggage animals needed to haul it. Any artillery
equipment such as ballistae or onagri which was carted by
the unit, may have been stored in this central area when not emplaced
along the defences, as urgency and need dictated.
- Praetentura - This area was the forward part of the camp
closest to the enemy, where the cream of the Roman soldiers themselves
were barracked. In a legionary camp this was where the first cohort was
encamped, together with the strongest of the other cohorts.
- Retentura - This was the rearward part of the camp which
housed the remaining cohorts; roughly half of the military force. If
there were any mounted troops (i.e. the cavalry contingent attached to a
Roman legion) they were generally placed in this area.
The areas within the camp were delimited by a number of roads:
- Via Sagularis - Known also as the 'intervallum road', this ran round the complete circuit of the camp within the defences.
- Via Praetoria - Led from the praetorium in the centre of the camp toward the enemy, bisecting the praetentura. The point at which this road pierced the defences marked the front gate of the camp.
- Via Decumana - This road ran from behind the praetorium in the centre of the camp towards the rear gate, bisecting the retentura.
- Via Principalis - This road ran in front of the praetorium in the centre of the camp at right angles to the via praetoria, separating the praetorium from the praetentura in the forward part of the camp. This road was usually continued at both ends through gateways in the defensive circuit.
- Via Quintana - This road ran at right angles to the via decumana, separating the praetorium from the retentura at the rear of the camp. This road often terminated at the via sagularis, without gates through the defensive circuit, unlike the via principalis.
The defences were pierced by gateways, usually four, at the termini of the major internal roads:
- Porta Praetoria - The main entrance to the camp at the terminus of the via praetoria,
was invariably placed in the centre of the defenses facing the
direction from which any potential danger would arise, e.g. a strategic
river-crossing, an enemy stronghold, etc.
- Porta Decumana - Was situated at the rear of the camp, opposite the praetorian gate, at the terminus of the via decumana. In some of the smaller forts, and a fair proportion of fortlets, this gate was not present.
- Portae Principali - These gates were situated at the sides of
the encampment, usually placed forward of the central point of the
defences, at either end of the via principalis; the gate on the right side of the camp being named the porta principalis dextra, the left-hand gate was, unsurprisingly, named the porta principalis sinistra.
- Portae Quintanae - These gates were often omitted, especially
in the smaller encampments. If present, they were situated in the sides
of the defensive circuit, again off-set from the centre, this time
towards the rear, at either end of the via quintana.
The dimensions and complexity of these defensive elements would
obviously depend on the size of the force the camp was intended to house
and the period of time that the encampment was planned to be occupied.
Camps, Forts and Fortresses
There are traditionally three main types of Roman fortification; the
Marching Camp, the Auxiliary Fort and the Legionary fortress. To these
three basic types there has been recently added a fourth classification,
that of Vexillation Fortress.
There are also smaller fortifications such as Fortlets, Signal
Stations, Light Houses and Watch Towers. Of these types, the Fortlets
may be viewed simply as small forts posessed of an unorthodox interior
layout and with no administrative buildings, but these latter classes
which, in general, are localised in their distribution within Britain
and often unique in construction, are therefore not included in this
We will deal here with the Four revised types of Roman Military Establishment;
- Marching Camps These are characterized by a single narrow
ditch and interior rampart, are generally rectangular in outline, but
can vary widely in size. These camps generally represent the
entrenchments made by a single Roman army unit for an overnight stop in
field conditions, when the enemy is close at hand and there is chance of
an attack. The size of the army unit on the march would obviously
dictate the actual dimensions of the camp.
- Auxiliary Forts These were generally rectangular or square in
outline, posessed of a substantial rampart and may have several ditch
systems. They were constructed at first mainly of timber but later in
stone, and housed troops from allied and Romanized nations, who would
become full Roman citizens on discharge, the Auxilia. These
troops were not as highly trained (or as well paid) as were the citizen
troops of the Roman Legions, and although they made their own Marching
Camps, their Garrison Forts were actually built by the legionaries.
- Legionary Fortresses These were, as the name implies, the
permanent strategic military encampments of the Roman legions, which
were occupied for any period between tens of years like Viroconium (Wroxeter) or even centuries like Eburacum (York). As they were built to house an entire legion, where the number of soldiers did not vary (i.e. c.5,200
legionaries), their size is fairly uniform at around fifty acres. Their
defences are massive, generally of stone, although the Fortress at Isca Dumnoniorum (Exeter) was occupied for only a short period and its defences were merely of timber construction.
- Vexillation Fortresses These were large encampments of a
fairly uniform size, which were built to house task forces comprised of
perhaps half a legion and several Auxiliary cohorts. They could be used
for either a single summer campaign like the Rhyn Park fortress in Shropshire, or used successively over a number of seasons like the fortress at Lake Farm in Dorset. Their purpose was therefore mainly tactical rather than strategic.
The Marching Camp
In a temporary overnight camp not intended for re-use after the force had moved on the following morning, the agger may be formed merely from the piled-up outcast from the fosse,
surmounted with a rough palisade of stakes thrust into the earth along
the top of this bank. The overall width and size would vary depending on
the number of men available for the task.
The Roman marching camp was constructed in the following manner:
- The area would be scouted and the best site chosen.
- The centre of the site would be marked by a flag; this would
preferably be placed at a point slightly higher than the surrounding
- The camp engineer would take sightings using a single groma -
a simple instrument which allowed the efficient sighting of
right-angles - placed at the designated centre, and the positions of the
intended gateways would be marked by other pairs of marker flags at
measured distances paced out from this central point.
- Upon the arrival at the camp site of the bulk of the force, each
unit would move to its assigned position within the marked-out area and
would dump its gear. The strongest and most experienced centuries would
be first, and they would march through almost the entire length of the
marked out area before turning aside and making camp; in this way the
most experienced troops were set to work on the defences nearest to the
- Every eight-man contubernium in each century would assign each of its members to different tasks:
- If the camp was made in hostile territory, a proportion of the force
would be used to form a defensive cordon around the remainder, who
would prepare the encampment.
- The bulk of the force would be used to construct the camp defenses,
usually comprising of a single ditch and an inner bank formed from the
ditch outcast, with a row of staves implanted in the top of the bank. If
there were sufficient men, the defenses may be more elaborate, perhaps
built of stacked turves.
- Whilst the heavy construction fell to the rank and file, under the
watchful eye of their centurions, some legionaries were excused the
dirty work and as a consequence were termed immunes (Latin immunis, free or
exempt from...). These would be required to perform the less arduous
tasks; clearing the camp interior, unloading baggage, erecting tents,
cooking dinner, tending horses, etc.
- The first half of the force would already be employed building the
forward part of the camp by the time the commander arrived and took up
position at the centre. He would probably begin with a meeting of all
centurions and officers to discuss any immediate defensive problems.
- During the time that it took the rearward half of the force to reach
the encampment, most of the defensive circuit would already have been
delineated by a bank and ditch.
The Auxiliary Fort or Campaign Fort
A tactical auxiliary fort, built to last at least one campaign season
guarding an important site, were built with more intrinsic strength and
solidity. The usual method from Caesar's time and throughout the 1st
century A.D. was to build a rampart of turf retaining
walls, but by the 2nd century the preferred method was to replace the
front facing or both retaining walls in stone:
- The site was surveyed using the same basic methods used for a
marching camp, described above, though probably with less haste, on a
more carefully chosen site.
- After accurately marking out the positions of the ditches, ramparts
and gates, work would begin by removing the turf from the areas
delineating the ditches and ramparts.
- The gathered turves would be stacked together forming two walls of
turf (or stone), each about a yard (0.9 metre) thick and separated by a
gap of around ten feet (3 metres).
- While the turf walls were being stacked, post-holes would be dug at
the corners and gaps in the ramparts, and choice timbers would be
erected to form the frameworks of corner-towers and gateways.
- The outcast from the ditches surrounding the fort would provide the
infill material for the interior space between the turf walls. The
processes of infilling and building-up the enclosing turf walls would be
going on simultaneously.
- The rampart may be strengthened or modified in a number of ways:
- If sufficient local timber was available - as was often the case in
Britain - the walls of the fort may be strengthened by the inclusion of a
timber lattice-work within the intermural space.
- If timber resources were plentiful the entire front of the rampart
may be faced with timber, perhaps part-replacing the outer turf wall.
- If the area was particulaly damp or prone to flooding, the entire
rampart may be built upon a raft of logs or stones, again, if locally
- In some particularly damp areas, the rampart would perhaps be built
of alternate laminated layers of sand and clay. Clay was also used to
line ditches in sandy soil, to prevent slippage.
- A turf rampart would be built almost vertically upwards for about
ten feet, and would be between twelve to sixteen feet wide at its base.
The top of the rampart could be anything between ten to four feet in
width, depending on the angle of slope imparted to the turf walls at the
front and rear.
- The top surface of the rampart was boarded over by a catwalk, and a timber palisade erected at the front.
- Turves for the defences were also removed from the areas delineating
the major roads in the interior of the fort, which were then surfaced
- The interior buildings would be the last things constructed, mainly of timber, though sometimes in stone, particularly the sacellum in the centre of the fort, which housed the regimental standards and the treasury; in many Roman forts, the sacellum was the only building made of stone.