The Feudal Age turns your settlement into something worth defending — and worth attacking. Archery ranges and stables open up ranged and mounted forces, walls and gates make defense a design problem instead of a prayer, and your economy is finally strong enough to support a real military.
This era introduces the counter triangle that shapes the whole game: spears hold the line against cavalry, cavalry runs down archers, and archers punish unarmored infantry. None of it is automatic — positioning and timing decide whether the counter fires.
Feudal is also when map control starts mattering. Scouting reveals where your opponent is investing, and a well-timed raid on an exposed gathering line can decide the next two ages before they happen.