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