Learn Croatian: Moramo okružiti neprijatelja prije noći. - We must surround the enemy before night.

The base verb is kružiti, which means to circle or orbit (an ongoing, imperfective action). Adding the prefix o- adds the meaning of encompassing or completely wrapping around a target. It also makes the verb perfective, meaning the act of surrounding the enemy is viewed as a single, completed goal.

Could I use the verb zaokružiti here instead of okružiti?

No, because they refer to different types of circling. Okružiti means to physically surround or enclose something, like an enemy, a building, or being surrounded by friends. Zaokružiti means to literally draw a circle around something (like an answer on a test) or to round a number up or down.

Why does the direct object neprijatelja end in -a? Doesn't the masculine accusative usually look exactly like the nominative?

That rule only applies to inanimate masculine nouns (objects). Because neprijatelj (enemy) is a living being, it follows the animacy rule: its accusative case takes the exact same form as its genitive case, which ends in -a.

The preposition prije requires the genitive case. Why does noći end in -i instead of the typical feminine genitive ending -e?