he easiest way to keep enemies away is to enclose your city in walls. Not even fine Roman walls, built of earth and stone, are immune to attack. A determined enemy will eventually break through even the best wall. But walls can be built as thick as you have the money and space for, and thick walls can take a very long time to penetrate.

Even a thin wall, easily battered down by a determined enemy, delays invaders from swarming into your town, or persuades them to attack a different location, which might lead them to a less important part of your city.

But walls are rarely built alone. Most cities place towers at regular intervals along their walls. Each tower can employ guards to patrol the walls, launching a volley of javelins towards any enemy who dares come within range. The guards also man a “ballista,” – a powerful catapult-like machine that fires heavy arrows – situated on top of each tower. A ballista is powerful, and extremely useful for taking out attacking elephants or chariots.

The guards must be trained at a barracks before they report for duty. Once they arrive at a tower, they man its ballista and patrol the walls, assuming that the walls are two (or more) segments thick. A single-thickness wall gives guards no space to walk along.

Tower guards won't abandon the city's defensive works even if invaders break through the walls. Although walls need neither road access nor maintenance, towers do need road access so the guards can find their way there swiftly.