You should be able walk back along the balcony opposite the one where you reclaimed your weapons and an artefact. You should find yourself near the prisoner you saw being beaten up. From there you should be able to make your way out. No levers required. If this is not the case, you have, indeed, found a bug. Try reloading from the last checkpoint.
By the way, the other prisoner you talked to is JRR Tolkien of "Lord of the Rings" fame
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