The Castle Age is the era most strategy players recognize on sight: stone fortifications, knights, and siege engines rolling toward walls. Castles and forts turn parts of your battlefield into territory that an unprepared army simply cannot take.
That permanence changes how you think. A castle placed well guards an expansion for the rest of the game; placed badly, it’s an expensive monument to a mistake. Siege workshops exist to make the question urgent — ballistae and siege engines mean no position is truly safe, and they punish players who believe walls alone win games.
Castle Age battles reward combined arms more than any earlier era: knights to threaten the field, spears to protect the siege line, archers behind, and a plan for what happens when the wall comes down.