Here is the full list of all 13 Fort Defense achievements.
Destroy 1000 enemies
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
Complete a level without using spells
Kill 5 enemies in one shot