Here is the full list of all 27 Fortify achievements.
Construct a lumbermill early in the game
Do not take any damage the first day!
Construct a quarry early in the game
Construct an iron mine early in the game
Use the catapult
Use the cannon
Have the spikes around castle or outpost deal the killing blow to an enemy
Light a tar field on fire
Kill an enemy with a burning tar field
Construct all four towers
Construct all buildings
Reach wave 5 in Arcade mode
Reach wave 20 in arcade mode
Reach wave 30 in arcade mode
Beat someone in a multiplayer match!
Finish North mission on Hard
Finish North mission on Impossible
Upgrade a unit to the next tier
Share your arcade score online
Share your mission score online
Heal one of your units with the surgery
Find the Bird hiding in the trees
Win 10 multiplayer games!
Win 30 multiplayer games!
Finish West mission on Hard
Finish West mission on Impossible
Destroy an opponent's outpost in a multiplayer match