Here is the full list of all 23 Fortified achievements.
Complete a map other than Easy Street without the base taking damage
Complete every map on normal difficulty
Spend more than $7,500 in any map other than Easy Street
Defeat the Juggernaut
Set fire to 10 enemies simultaneously
Destroy an Annihilator before it deploys
Reach level 20
Reach level 20 with all player characters
Complete any map on hard, other than Easy Street without taking damage
Complete any map other than Easy Street without any structure kills
Destroy 10 enemies with the Rifle in 5 seconds
Destroy 5 flying enemies in 5 seconds with the shotgun
Complete any map other than Easy Street using only infantry
Destroy 20 enemies within 1 second
Freeze 10 enemies within 1 second
Destroy 50 enemies using Tesla Coils in any single map other than Easy Street
Throw 10 enemies at once with the Cryo Blaster secondary attack
Survive 10 waves of Invasion Mode
Defeat the Stalker
Unlock every structure and weapon
Complete every map on hard
Complete all maps on insane
Survive to wave 20 in Invasion mode on hard