A
The speaker portrays himself as the girls' enemy and victim in their pretend game of raiding the castle.
B
The speaker expands this castle imagery to beyond his study, symbolic of the girls' invasion into all aspects of his home and life.
C
The speaker expands this castle imagery until it includes himself, wherein his heart is a round-tower dungeon where he will always keep his love for his daughters.
D
The speaker expands this castle imagery until it includes himself as parts of the castle, implying that he wants to remain guarded against his daughters' affection.