围城工事

例句：

1. 1. The city was surrounded by a ring of walls that protected it from invaders.
   这座城市被一道城墙包围，保护它免受侵略者的攻击。
2. 2. The ancient fortress was an example of a well-constructed encircling fortification.
   这个古老的堡垒是精心设计的环绕防御工事的一个例证。
3. 3. The army built several defensive positions around the city to prevent the enemy from invading.
   军队在城市周围建立了几个防御位置，以防止敌人入侵。