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Currently Kingdom 'Heavy Armor' unlocks 5 techs: Expert Armor, Expert Cutting Weapons, Expert Blunt Weapons, Refined Defense, and Refined Weaponry. Refined Defense should unlock under Expert Armor, and Refined Weaponry should unlock under Expert Cutting + Expert Blunt. This is to make the branch consistent with all other tech branches.
The Karrazan doesn't become available for Kingdom troops upon researching Expert Cutting Weapons, and the Elemental Karrazan doesn't become available for troops after researching Arcane Weaponry. They both do become available in the Store for Champions.
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There are three magic items available to Empire that are not available to Kingdom: Amulet of Clarity, Crystalline Stone, Hate Stone. Conversely there are no magical items available to Kingdom that are not also available to Empire. This might not be a bug, especially in the case of the Hate Stone, but for the Amulet of Clarity at least it sort of feels like an oversight (no pun intended; the amulet increases sight; I crack myself up). EDIT: OK after double checking Kingdom gets the Amulet of Warding and Empire doesn't, so there is a balance for the amulets at least.
The last time weapons were rebalanced the level 1 quest item "Sword of the Sovereign" was not changed, making it the most powerful one-handed sword in the game right now (barring Pariden's Sword from the Master Quest). Which honestly I think is fine and not overpowered (it is ATK 11), but probably wasn't intended.
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'The Ties That Bind' in the Kingdom Diplomacy has no effect; the diplomatic benefit for mutual family and grandchildren stay at +2 and +1 respectively. The text of the tech says that the player will receive greater benefit from arranged marriage families.
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The quest 'Alzor's Armor' rewards you with an item called 'Azur's Armor' - having it spelled consistently would be nice 
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The 'Tyrant' from the dragon's egg quest end battle spawns as a random monster on the world map. He has no attack value and the dominate ability.