Here is the full list of all 56 Strategy & Tactics: Dark Ages achievements.
Hire the General of the Black company mercenary squad
Recruit squad Union Pikemen
Recruit squad Landsknechts
NEW_ACHIEVEMENT_DESC_2_27
NEW_ACHIEVEMENT_DESC_3_3
Apply the Light Armor Upgrade
Hire the General of the Seventh sons mercenary squad
Hire the General of the Widowmakers mercenary squad
NEW_ACHIEVEMENT_DESC_2_26
Apply the Training Ground Upgrade
Apply the Chain Mails Upgrade
NEW_ACHIEVEMENT_DESC_3_6
NEW_ACHIEVEMENT_DESC_2_25
NEW_ACHIEVEMENT_DESC_3_7
NEW_ACHIEVEMENT_DESC_2_30
NEW_ACHIEVEMENT_DESC_3_2
Apply the Sturdy Harness Upgrade
Apply the Sharpened Swords Upgrade.
Win the Empty Throne scenario.
Recruit squad Baronial Squad
Recruit squad Northern Archers
Recruit squad Swordsmen
Recruit squad Free Riders
Recruit squad Militia
Complete all main and additional goals in any scenario.
Recruit squad Peasants.
Hire the General of the Red slashers mercenary squad
Recruit squad Union Crossbowmen
Recruit squad Dart Throwers
Recruit squad Sun Phalanx
Recruit squad Town Guards
Recruit squad Twin Rivers Archers
Recruit squad Union Musketeers
Recruit squad War Elephants
Recruit squad Twinrivers Templars
Apply the Improved Bows Upgrade
Apply the Penetrating Arrows Upgrade
Apply the Practice Range Upgrade
Apply the Stables Upgrade
Apply the Sturdy Shields Upgrade
Apply the Breastplates Upgrade
Apply the Lightweight Armor Upgrade
Apply the Stronger Skins Upgrade
Apply the High-Quality Steel Upgrade
NEW_ACHIEVEMENT_DESC_2_28
NEW_ACHIEVEMENT_DESC_2_29
NEW_ACHIEVEMENT_DESC_2_31
NEW_ACHIEVEMENT_DESC_3_0
NEW_ACHIEVEMENT_DESC_3_1
NEW_ACHIEVEMENT_DESC_3_4
NEW_ACHIEVEMENT_DESC_3_5
NEW_ACHIEVEMENT_DESC_3_8
NEW_ACHIEVEMENT_DESC_3_9
NEW_ACHIEVEMENT_DESC_3_10
NEW_ACHIEVEMENT_DESC_3_11
NEW_ACHIEVEMENT_DESC_3_12