n.[C]

1. （护城）城堡；堡垒，要塞
2. 安全的地方；避难处
3. （军舰上的）炮塔
   The invading warplane dropped a bomb which destroyed the citadel of the frigate.

   来袭的飞机丢下一颗炸弹，炸毁了军舰上的炮塔。

n.

1. a fortress protecting or dominating a city.
2. a meeting hall of the Salvation Army.