Here is the full list of all 12 Siegecraft Commander achievements.
Crashed air units into each other
Shot down an air unit with a keg
Killed a tower with a full power trebuchet shot
Caught a cow with a dragon
Destroyed 10 units with a single keg
Destroyed a tower with a building projectile
As a tower is being destroyed, fire a projectile and destroy an enemy tower
Get 5 tower kills by destroying the tower it is linked to
Changed a cow into a mad cow
Chained 4 Towers with a single Hex Curse
Hit 8 friendly units with the haste spell
Hit 10 enemy units with the freeze spell