Here is the full list of all 13 Fort Defense achievements.
Earn 120 stars
Lose three times in a row
Complete a level with one heart remaining
Get a magical ability to the maximum level
Buy all upgrades for one cannon
Use 200 spells
Complete all levels of an episode with three 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
Kill 5 enemies in one shot
Destroy 1000 enemies