Yuri_G
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...As a result, first a red player appeared (in the form of a turn indicator), on the next turn, we finally captured the first garrison (a human army arrived, and there were 10+ cavalry units), and on the next turn, Briara entered my territory, and the "Hold the Border" quest was completed (although the "Capture Garrisons" objective remained incomplete)...
Well, that's correct. The "Hold the Border" quest implies holding it until the red player arrives. And unfortunately, after Briara's death, it's no longer possible to capture the garrisons.