长城曾是抵御外敌的重要屏障。

Chángchéng céng shì dǐyù wàidí de zhòngyào píngzhàng.

The Great Wall was once an important barrier against foreign enemies.

这片森林是防止沙漠扩张的天然屏障。

Zhè piàn sēnlín shì fángzhǐ shāmò kuòzhāng de tiānrán píngzhàng.

This forest is a natural barrier against desert expansion.

我们需要建立一个有效的安全屏障。

Wǒmen xūyào jiànlì yī gè yǒuxiào de ānquán píngzhàng.

We need to establish an effective safety barrier.