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