Here is the full list of all 15 Castra achievements.
Conquer your first territory
Win a game of conquest
Reach 300 supply
Win one game with every faction
Lose your capital
Win a game without losing a single territory
Win a game in under 10 days
Control 100% of territories on the map
Win 25 games
Build 100 infantry units
Defeat 500 units in battle
Take an enemy capital
Win 5 battles in a row
Use every attack card once
Win a game on every map, with every faction