After 12 hours of gruelling battling time with the Barbarians, we need to retake capitals that were contested and rebuild the regional structures that were levelled down.
Regional building upgrades need silver, but do not just collect silver. Gathering silver takes a very long time and not so much of them available from farming dead cities.
So the best thing to do is gather from farms because it is the fastest of gathering then trade them at trading posts. Try to look for positive trades as high as possible, meaning that you give little food and get more silver. The highest I came across was 29%.