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