For castles under Lv. 30, each upgrade will increase castle level by 1; after reaching Lv. 30, each upgrade will get 10% progress, and castle level will be increased by 1 after the progress reaches 100%; castles of Lv. 31 to Lv. 36 are called Prestige 1 to Prestige 6.
There are 10 preconditions to upgrade a Prestige 5 castle to Prestige 6:
10%–The number of completed quests: 600
20%–Plunder resource: 20M
30%–Lord Level: 40
40%–Science Power: 2M
50%–1 piece of a Gold Equipment Set (1 piece of gold equipment from any set is OK)
60%–Number of soldiers: 1.5M
70%–Siege Victories: 2000
80%-Total number of healed soldiers: 500K
90%–the number of killing soldiers: 2M
100%– Conquest victory: once (requires only the result of conquest stage, not the result of the whole the Strongest Kingdom; please note that you need to finish upgrading the previous 90% before the conquest evaluate; it’s OK if you haven’t finished upgrading to 90% when the conquest stage begins)
P.S. If Lords cannot reach the above preconditions, you can skip them with Gold: the first time of skipping the precondition with Gold will cost you 100,000 Gold; after that, the Gold consumption will increase by 100,000 each time; skip the last precondition (winning the conquest stage once) will cost 100,000 Prestige Badge.
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Game Guide for Clash of Kings