Fountain! Good name.
I think the trick for balancing the Fountain should be twofold:
-A high resource cost
-Some other hazard to manage in the late-game, such as bandits (ie: The more food production you have, the more likely one or more will be occupied by stealthy bandits. They won't destroy the tile, but instead just prevent it from generating anything until the player notices and clears them. The GUI element for the presence of bandts should be very subtle (kind of like how the fires right now are hard to see at night). In fact, having the bandits only be active during the night could be an important part of differenciating between the Day and Night cycles)
An additional suggestion: 200 (or more) Faith to extend the duration of a session before the eruption. Maybe even one with an ever-increasing cost.
200, 225, 275, 350, 450, et cetera
I'm definitely thinking about other hazards and maybe even an event system.
I initially planned for extending the duration of the session to cost population, as faith will also be used for a new type of cards, but yeah, the price will increase the more you use it, and it could only be used a certain amount of times to prevent infinite matches.