Here is the full list of all 27 Siege Wars achievements.
Complete one level with any score
Kill an enemy from Sand Mage Guild
Defeat Yellow Mage
Get rid of Green Mage
Liquidate Sky Mage
Slay Frost Mage
Destroy Fire Mage
End the life of Volcano Mage
Make away with Violet Mage
Brush of Black Mage
Assassinate White Mage
Complete all levels in Wastelands location
Complete all levels in Black Marsh location
Complete all levels in Icy Plains location
Complete all levels in Volcano location
Complete all levels in Citadel location
Upgrade your Catapult weapon up to level 8
Upgrade your Bombard weapon up to level 8
Upgrade your Ballista weapon up to level 8
Upgrade your Arch-Ballista up to level 8
Cast Shield spell
Cast Heal spell
Cast Earthquake spell
Complete 10 levels for 1 star or more
Complete 25 levels for 2 stars or more
Complete 50 levels for 3 stars
Get magic amulet from a level