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