Defensive Tactics (Civ6)/Civilopedia

Besides cowering behind walls when the barbarians came, the first defensive tactics consisted of packing armored men in as tight as possible, with lots of sharp spears pointing outward to discourage attackers. The first to adopt such a phalanx formation were the Sumerians around the third millennium BC. Their densely packed infantry were armored (helmet, breastplate, and greaves) and armed (with nine-foot spears and round shields that could be overlapped). For the next few thousand years defensive tactics didn’t change much, save that archery made better armor ever more important.
As with so many things military, Rome improved upon the defensive tactics employed by most of their competitors. The Romans pioneered the manipular legion sometime around 300 BC, giving their formations more flexibility and support; deployed in a triple line with gaps between the maniples, the legion could react rapidly to any threat. In front before battle, elite skirmishers (the velites) covered the deployment of the legion; and during battle cavalry and auxiliaries protected the flanks. The thoughtful Romans developed defensive tactics to counter every threat conceivable: the testudo to protect from missiles, the saw to quickly plug gaps in the line, the orb for formations that had been cut off, and Fabian tactics to avoid battle when at a disadvantage.
In the Middle Ages such cleverness was largely thrown over for simply charging at the enemy and hacking away until one side or the other had had enough. The difference in tactics between Viking berserkers and European knights is miniscule, as military thinking focused on the offensive and largely forgot the defensive (save for those towering castle walls and masses of archers). That is, until gunpowder made head-long charges a dicey proposition again. As firearms became cheaper and more effective, requiring little training to be used in defense, gunpowder dominated the battlefield. And the only real defensive tactic became to spread troops out, to use camouflage, and to dig down.