The Vault of the King is a secondary resource, unit production, and upgrades building of Lothlorien. It can be constructed on one of the build plots of the Mirkwood Outpost. While it can't directly train units, it allows you to recruit Palace Guards at the Mirkwood Outpost's main citadel.
Forged Worker's Tools: Upgrades the building to rank 3. At rank 3, internal economy buildings produce 20% more resources and gain 1000 additional hit points.
The upgrades can be researched from the Forge
- 5 economy structures or less: 100% production
- 10 economy structures: 84% production
- 15 economy structures: 69% production
- 20 economy structures: 56% production
- 30 economy structures: 49% production
- 37 economy structures or more: 44% production
King's Guard - The Vault of the King also provides a reduction to the cost of Palace Guards:
- 2 Vaults of the King: -20%
- 3 Vaults of the King: -30%
This building can also research four upgrades that affect the other Mirkwood Outpost add-ons and units.
Wine from Dorwinion - Increases the production of wine cellars by 25%. Cost: 1000
Protect the Borders - Troopchambers will be manned with two elite archers of the king that shoot at nearby enemies. Cost: 500
Captain of the Watch - Each Vault of the King is guarded by two individual Palace Guards and a Banner Carrier. The Banner Carrier grants nearby allies + 25% armor. Cost: 500
Thranduil's Mobilization - The Mirkwood citadel can now use the "Mobilization Order" ability, which speeds up recruitment of all Mirkwood troops for a short duration. Cost: 750
Although this building can't produce units, it is the main prerequisite for the player to be able to train Palace Guards in the Mirkwood Outpost. The other prerequisite is that the Forged Blade upgrade has been researched.
At least one should be built if you plan to focus on Mirkwood, as it provides upgrades ranging from guards for the outpost and improved resource production to a ability that enables the Mirkwood units to be recruited quickly, helping to overwhelm the enemy with the armies of Thranduil.
The building provides a discount for Palace Guards if two or three are built, making building more than one really useful against cavalry-centered strategies or factions, like Rohan. Lothlorien is a faction that is generally weak against cavalry, but the Palace Guards are amongst the best pikemen ingame, so a cheaper price for them can turn the tide against cavalry.