As it stands, the citadel upgrade which provides the team with extra gold per second is far too weak, considering its cost.  Specifically speaking, the late war rank upgrades cost too much for their benefit.

The first level of this feature is unlocked at war rank 3, and costs 1,800 gold.  This provides a benefit of +2 gold / second to the entire team.

In other words, it will provide 1,800 gold to each member of the team, after 15 minutes. 

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Fair enough, you say, since most games tend to last more than 15 minutes after reaching the third war rank. However, there are additional ranks to this skill, and their cost is much more obscene -- however, they also only provide an extra 2 gold per second. 

Late in the game nobody is going to spend over 3k gold to earn another measly 2 gold per second upgrade for their team.  Change this citadel upgrade to that it improves gold earned by a percentage.

For example, the first upgrade could provide 10% extra gold, the second upgrade could provide 20% extra gold, and the third upgrade could provide 30% extra gold (or whichever, but a percentage gain to total earnings is the key to this change).  This way, the demigods who spend this gold on their citadel will reap higher returns when they manage to kill opposing demigods.