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