Here is the full list of all 7 Donjon Defense achievements.
Collect an army with 80 arquebusiers
Collect an army with 40 Da Vinci's machine guns
Build a castle consisting of 100 stone walls
Build a castle consisting of 100 wooden walls
Collect an army with 40 cannons
Build a castle consisting of 100 metal walls
Collect an army with 40 crossbowmans