Here is the full list of all 13 Fort Defense achievements.
Complete a level with one heart remaining
Lose three times in a row
Get a magical ability to the maximum level
Buy all upgrades for one cannon
Use 200 spells
Earn 120 stars
Destroy 10 000 enemies
Complete a level without using spells
Complete all levels of an episode with three stars
Allow only one enemy to attack the base's gates throughout a level
Destroy 5000 enemies
Kill 5 enemies in one shot
Destroy 1000 enemies