Learn Finnish: Kuningas sanoi, että vihollinen on julma, mutta kaupunkia ei liene tuhottu. - The king said that the enemy is cruel, but the city was probably not destroyed.

The structure ei liene tuhottu is the potential mood passive perfect negative. It is formed with the negative verb ei, the potential connegative of the verb olla (liene), and the passive past participle of the main verb (tuhottu from tuhota). It expresses that something was 'probably not' done.

Why is kaupunkia in the partitive case here?

Kaupunkia is the object of the verb tuhota (to destroy). In Finnish, the object of a negative sentence must always be in the partitive case. Even though the affirmative version of this sentence might use a nominative object (kaupunki lienee tuhottu), the negative ei forces the partitive.

Why does the sentence use the present tense on after the past tense sanoi?

This highlights a major difference between Finnish and English. English shifts tenses back in reported speech (The king said that the enemy WAS cruel). Finnish usually keeps the original tense of the statement if the fact is still true or viewed as a general state, so on julma is perfectly natural here.

Is the potential mood common here, or is there an easier way to say 'probably not destroyed'?

The potential mood is quite formal and mostly found in literature or news. In everyday spoken or written Finnish, you would more likely use the standard negative passive perfect with an adverb like varmaan (probably), resulting in: kaupunkia ei varmaan ole tuhottu.