Here is the full list of all 27 Fort Triumph achievements.
Win a Skirmish with every faction.
Finish Act III on Normal or higher
Finish Act III on Classic or higher
Finish Act III on Legendary
Save an unappreciative ally.
Finish Act II
Defealt Galigarn, an outspoken opponent of mattresses.
Finish Act I
Have a hero with 3 cross-class skills