Quoted from my topic, these are some ideas that have not been shared here yet.
- Buildings provide 8% gold of their wood cost when built.
- Each Market generates 0.25 food per second.
- Units generate 20 gold when killing aggressive wild animals like boars and wolves.
- When trading units gather gold, directly add 50% to the stockpile and carry only the another 50% back.
This means that a certain aspect of civilization (in your example, farming) will get punished as the game progresses. I don’t think this is a friendly design.
If you just mean that the enhancements before the Feudal Age will not disappear after the Castle Age (in your example, the +30% farming would not lose in the Castle Age), then you can simply list it as two bonuses to avoid misunderstanding.