Here is the full list of all 19 Castle Invasion: Throne Out achievements worth 1,000 gamerscore.
Complete a Night Level
Defeat the King's Dragon
Collect 3 Stars in a Time Level
Unlock the Spear
Defeat 50 Dwarves
Complete Level 1
Defeat the King's Troll
Defeat the Evil Archer
Defeat the King's Commander
Defeat the King!
Fully Upgrade the Bow
Fully Upgrade the Spear
Fully Upgrade the Crossbow
Unlock all Upgrades!
Hit Every Enemy Type With the Spear
Knock a Dwarf Off a Hog
Collect 50 Stars
Collect 100 Stars
Collect 150 Stars