The Legion's raw strength and ability to create forts makes it an invaluable tool in early-game expansion. Capturing multiple cities in a single campaign becomes much easier, since forts can be constructed immediately around a captured city. It is critical, however, to refrain from upgrading at least one Legion and keep its single build charge intact until late in the game, so that it can be used to clear nuclear contamination and thus obtain the "Missed That Day in History Class" achievement.
The legion was one of the most efficient and effective fighting forces in history. A fully-equipped, fully-staffed Roman legion could march across any terrain, laying a road as it did, built a fortified camp at the end of it, and then take any foe or take any wall. During the Empire a legion was composed of infantry, recruited exclusively from Roman citizens, along with the cooks, armorers, engineers, artillerists, medics and command staff (as well as the usual camp followers and sutlers) necessary to operate in the field for years at a time. The light infantry, archers, scouts, and cavalry were generally auxiliaries, provincial non-citizens (those who survived to be honorably discharged gained citizenship) assigned to a legion as necessity dictated. Extremely disciplined, the legion’s heavy infantry were armed with pila (throwing spears) and gladius (short sword) and conquered most of the known world at the time.