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