A ditch dug around the castle, often filled with water✓Правильный ответ
A raised mound of earth on which the wooden tower stood
A covered passage running under the castle wall
A walled yard where the garrison kept its horses
Объяснение
The defence worked by forcing attackers down into an obstacle and up out of it again before they could reach the foot of the wall, and where the ground allowed it the cut was flooded to make that harder still. Describing it as a mound fails because that similar-sounding word names an artificial hill raised above ground level, which is the opposite of an excavated ditch.