The Romans & The Antonine Wall.
The Wall was constructed around 142 AD, is 61km (38miles) long and covered the narrowest part of Lowland Scotland between Carriden on the Forth and Old Kilpatrick on the Clyde. Its main defensive feature was a turf and earthen rampart 3.5 meters high built on a stone foundation with a wooden palisade on top and a ditch in front. This ditch was about 12 meters wide and 4.2 meters deep. Prior to building, the area was well surveyed in order that the best use of natural features and contours was made so that this lessened the actual work required. This can be seen particularly well at Croy Hill where the North facing side was a natural barrier where less work was required.
Unlike England, Scotland was never considered part of the Roman Empire although the Romans did advance into Scotland several times during their 300-odd years in Britain.
Their first foray north was made in 79 or 80 AD by Cnaeus Julius Agricola from Carlisle, reaching as far as Perthshire. Later expeditions in 82 and 83 established forts as far north as Aberdeenshire and the following year Agricola's forces defeated the native Caledonii at Mons Graupius. However, most of his forts were abandoned shortly after and by 118, the effective limit of Roman rule was marked by Hadrian's Wall (named after the emperor Hadrian), a defensive barrier running across the north of England between the Tyne and the Solway.
During the 140s, the Romans tried to move their border northwards and built a new defensive barrier, the Antonine Wall, between the Forth and the Clyde. For the next forty or fifty years, the Romans regularly occupied and abandoned this position in favour of the security of Hadrian's Wall.
Between 208 and 211, the Emperor Septimus Severus conducted a major campaign against the Caledonii and other tribes from major camps based around the Tay and Angus. When Severus died in 211, the Romans retreated to Hadrian's Wall again.
The final incursions came a century after Severus, but this time, the Picts (a confederation of tribes based north of the Forth) fought back and by 367 had overrun Hadrian's Wall which the Legions finally abandoned in 400.
When Did the Romans Invade Scotland?

Historically it was believed that Gnaeus Julius Agricola, following his appointment as governor of Britain in late 77AD, invaded Scotland after a campaign in Wales. The primary source for this story is the Roman writer Tacitus. But as Tacitus was the son-in-law of Agricola, some historians believe that he played up the role of his father-in-law.  Older sources claim that Agricola was the first Roman to advance beyond present-day Perth and build the Gask Ridge, a series of wooden forts and watch-towers, around 80AD. 

However, Manchester University archeologists now conclude that the forts were built as early as 70 AD, during the rule of Petilius Cerealis. If this is the case, they are not only Britain's oldest frontier outposts but pre-date similar fortifications in Germany.  The watch towers crossed 20 miles of Perth and Stirlingshire and were rebuilt, sometimes more than once. Agricola was probably responsible for some of the reconstruction. Archaeologists have also shown that there is evidence of farming around this time, suggesting that there was a period of relative peace. It is thought that the forts may have been constructed to protect the Romans' new-found allies from invasion by the Caledonii, further north. The Caledonii were decisively defeated by Agricola at the Battle of Mons Graupius (site unknown but possibly near Inverurie in Aberdeenshire) in 83AD, where up to 10,000 Scots are believed to have died in one of the bloodiest battles ever fought on the British mainland.
The Purpose of the Sites .
In the early Empire forts were bases that offered security for their garrisons and their equipment. In wartime the enemy was fought in the field  At other times the garrison would have patrolled well beyond the frontier to support allied tribes and gather intelligence. Until the end of the Flavian period the army in Britain spent the summer almost continually campaigning in enemy territory as the province was expanded into what is now the Scottish Highlands.  Units built marching camps to provide shelter at night for their tents and, once an area was conquered, a network of turf and timber forts roughly a day’s march apart. In the pre-Flavian period, before the legions had established their permanent fortresses, they built large forts either to provide parts of army groups (legion and auxiliaries) with a long-term home or as a summer campaign base (aestiva) or winter quarters (hiberna). These forts, known as vexillation fortresses, are found mainly in the Midlands and southern Britain.
First and Second Century Forts.
Newly conquered areas were controlled by timber and turf forts approximately a day’s march apart. In Britain this phase lasted until the mid 80s AD. Advances in the mid 2nd century and the early third century proved to be temporary or were short punitive campaigns. However at most forts the evidence is of only one unit, but it is evident that often the whole unit was not based there. During the Antonine occupation of what is now lowland Scotland great use was made of small forts (fortlets) that lacked a headquarters building or the space to hold an entire unit. It is assumed that regiments would have had a headquarters fort whilst manning a group of surrounding fortlets.
Legio VI Victrix.
A Cohort of the 6th Legion manned the Fort at Croy under the command of the Roman Tribune Fabious Liberalis.
 It is also interesting to note that at Bar Hill there was a detachment of   Syrian Archers, Cohort Hamiorum Sagittariorum. This unit of Hamian archers was raised in Syria. In Hadrian's time it was the garrison of Coarvoran.                        It left to  become the garrison at Bar Hill during the second period of occupation of the Antonine Wall, returning to Carvoran in the reign of Marcus Aurelius
(ca. 163-166). 

 The 6th Legion had a long and exciting pedigree. It served with Julius Caesar in Egypt and fought at the battle of Actium (against Anthony and Cleopatra). Later it served in Spain and after this in Germany. Its association with Britain began when it was drafted in by the Emperor Hadrian in about 120 AD to assist in building his famous wall. Originally stationed at Newcastle, its main HQ was York. The building of the wall was a joint venture alongside Legio XX Valeria Victrix and Legio II Augusta. Twenty years later their joint building skills were again employed in building the Antonine Wall in central Scotland. From this time onwards Legio VI Victrix were directly involved in British war campaigns and in internecine warfare within the Roman Empire. This included rebuilding Hadrian's Wall in the early third century and re-establishing control after the 'Great Barbarian War' of 367 AD. The withdrawal of Roman forces from Britain in 410 AD left a shadowy VI Victrix at York, but its illustrious career had come to an end.   
In the Roman army, the commanding officer of a legion was called the Legate.  He was assisted by a deputy called the Camp Prefect, and a staff of six senior administrative officers called Tribunes. The original function of the Tribunes was to spread the call to arms and to ensure that the citizens rallied to the Eagles in time to march and fight. Later, the Tribunate became more of a political tenure, a training ground for young noblemen waiting to go into the consular or civil services.  Whenever a Tribune chose to distinguish himself militarily rather than serve his time administratively and get out, his success was almost preordained.