桥头堡

例句：

1. 1. The bridge is the only way to cross the river.
   这座桥是过河的唯一方式。
2. 2. The fortress was built to protect the city from invaders.
   这个要塞是为了保护城市免受侵略者的侵害而建造的。
3. 3. The stronghold of the rebel army was located in the mountains.
   叛军的要塞位于山区。