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