Questions & Answers about Milites arcem defendunt.
- milites = soldiers
- arcem = citadel, fortress, or stronghold
- defendunt = defend or are defending
So the sentence means The soldiers defend the citadel or The soldiers are defending the citadel.
Milites is in the nominative plural, which is the case normally used for the subject of a sentence.
Also, the verb defendunt ends in -unt, which means they. That matches a plural subject, so milites is the group doing the action.