4 sentences

1. 将军奉命镇守要塞。

   jiāng​jūn fèng​mìng zhèn shǒu yào​sài.

   The general was ordered to guard the fortress.

   Source: Dong Chinese Dictionary

2. 战士们日夜在要塞守卫把关。

   zhàn​shì men rì​yè zài yào​sài shǒu​wèi bǎ​guān.

   The soldiers guard the fortress day and night.

   Source: Dong Chinese Dictionary

3. 此地形势险要，是一个军事要塞。

   cǐ​dì xíng​shì xiǎn​yào, shì yī gè jūn​shì yào​sài.

   The terrain here is strategically critical; it is a military fortress.

   Source: Dong Chinese Dictionary

4. 这座城市曾是一个重要的军事要塞。

   zhè zuò chéng​shì céng shì yī gè zhòng​yào de jūn​shì yào​sài.

   This city used to be an important military fortress.

   Source: Dong Chinese Dictionary