Learn German: Die Mauer wurde schließlich zerstört.

In German, the passive voice for an action (the act of the wall being destroyed) uses werden as the helper verb, which becomes wurde in the simple past. If you said Die Mauer war zerstört, it would mean "The wall was already in a state of being destroyed." Using wurde focuses on the actual event taking place.

Why is zerstört at the very end of the sentence?

German uses a "sentence bracket" structure for the passive voice. The conjugated helper verb wurde takes the second position in the sentence, and the main action word, the past participle zerstört, is pushed to the very end.

Why doesn't the past participle zerstört start with ge-, like most past participles?

The infinitive verb is zerstören (to destroy). Verbs starting with the inseparable prefix zer- never add a ge- in their past participle form. They just take the normal ending, which is -t for regular verbs like this one.

Could I use endlich instead of schließlich for "finally" here?

You could, but it changes the tone. Schließlich means "finally" in the sense of "eventually" or "in the end" after a sequence of events. Endlich expresses an emotional relief or impatience, like saying "at long last!"