Building a Castle in or near the opponent’s base, usually as soon as you reach the Castle Age. A Castle costs 650 stone, has 4,800 hit points and shoots arrows, so it denies the area around it and trains unique units right where the fight is.
The opponent must answer it with siege (Rams, Trebuchets), by fighting its villagers before it finishes, or by giving up the resources near it. Stone mining in Feudal Age is often the first sign that a castle drop is coming.