In a current game on Yucata.de (http://www.yucata.de/en/Game/CasusBelli/4606422), both my opponent and I have tried to build gold producing cities and failed. We have a total of 10 gold cities on the board, which doesn't seem like it would trigger a limit and we can't find anything about it in the rules (http://www.yucata.de/en/Rules/CasusBelli). Is there a limit to the cities you can found to produce each resource?
The rules on Yucata state that when founding a city: "The player takes a city token out of the bank’s supply (marble, iron, or gold), and puts it at the place of the newly founded city on the board." It doesn't say anything about limits on these city tokens.