People romantically (har) think walls were there to keep the enemy out. It's actually to do the opposite, keep them in. They're chokepoints - takes a long time for even an organized body of people to cross an obstruction. But an unorganized horde? It's a death trap.

Hence the Roman engineering solution to this problem, the Roman roads. They could put a large organized army anywhere within their borders in an insane amount of time, camps always built on the march in the same exact configuration, with predictable roads and ditches. Though restructured to a point historians call fragile, Rome was still a bastion of engineering and organization. There was nowhere to run to and a price to pay.

In those days battles were always a matter of frustrating chase and escape. And once an engagement was forced, the Roman fighting style was terrifying and as equally organized as their infrastructure, nothing like a Hollywood confrontation with copied and pasted CGI fighters.

Lanes of soldiers in a checkerboard pattern called quincunx, timed by a whistle. Enemy faced a human chainsaw, basically. Imagine being their enemy, standing at the forefront of a ragged well-fed line with consistent kits and gear, each man carrying two spears, a sword and a shield.

Whistle blows, and the front line guys sprint at you full speed. At forty yards out they throw their light spear with a barbed tip and it gets stuck in your shield. Guy is still sprinting full speed at you. At ten yards out he throws the heavier spear and it too gets stuck in your shield. He shield slams you, hacks at you a few times unsuccessfully. You survived.

Then the whistle blows again.

Your shield feels noticeably heavier with two spears stuck in it. Guy turns around and runs back and there's another guy sprinting at you, about to throw a spear. You've just now realized first-hand how the Roman Imperial army fights.