You've signed up - though you aren't quite sure you've learnt to read or write, and they bring you here. Why didn't they conquer everywhere? Though they did go beyond the Wall. Not simply a frontier, just as much a political control, to ensure the natives didn't get too restless if not quite accepting the Roman way of life. Ten minutes march from the Roman Army Museum to the Wall itself - where the troops weren't Roman Legionaries but auxiliaries from provinces who had accepted the Roman way of life - even Northumbrian winters.
The Roman Army Museum has their own Soldier, Soldier. Aquila an ambitious young officer leads his men from pretty well anywhere except Rome. They moan, gripe, gamble, stand up to and for each other like soldiers forever and a day. They are professionals.
Rome's power lies in its standing army. You can fight 24/7/365 - the Romans also did everything to time. Marched to a beat, sentry-watched to the hour. The Legions' combat technique and equipment still hold good today. They are professionals, they are qualified to bear arms, they do not have to tend to farms and families, just fight when called for. The last thing you do is confront them head on.
So why did Hadrian need to build a Wall? Good question, and you'll find many answers...it seems to have worked, changing with changing conditions.
Carlisle is - or was - an army town. See the barrack blocks inside Carlisle Castle, rectangular in shape and arrangement, a model of every Roman fort in the empire. Those trained in Carlisle served throughout the empire - India, Africa, Near and Far East...even today we are a home for heroes from Afghanistan. And when Legionaries, Auxiliaries or Squadies go out on the town remember each is a mother's son.
Uniforms on horses are special.Trooping the Colour, mounted police at a soccer match. they command and control, add to ceremony. The Cavalry Helmet at Vindolanda would doubtless be a commander receiving the salute: imperial splendour leaving locals in awe.
Roman cavalry were helicopter gunships of their day. You might outrun infantry, not an earthly from those on horseback. just as latterday professional armies developed different regiments - hussars, lancers, cuirassiers, dragoons - so too the Romans: mounted archer auxiliaries from Dacia were stationed on the Wall. Locals had horses too, but probably not that many nor bred, trained and kept to fight, and fight in specific ways - a horse-rider isn't a cavalry man.
Where were they garrisoned? The Romans built specific cavalry forts at specific places. Chesters is the most well known and has the best preserved bath house along the Wall, but Birdoswald was orginally for cavalry too. It's no accident they're both next to the Wall's two bridges - a lovely walk down and across to Gilsland. These two bridges transform the Wall from three fronts to one, cavalry regiments located to exploit gaps. They knew what they were doing, the Romans.