Collecting and assuring a high castle level requires a good castle base structure with good buildings and good resources for the necessary income. All resources has a role in helping the castle grow when you level or upgrade structures your resources will expand higher with every level, but if you decide to buy or get golden age your resource output will increase dramatically. This condition is only for players who are willing to buy or get gold to get this option money is required.
The castle base structure is very important to have a good upkeep into maintaining a high level characters more resources and supplies are required, as far as talents go pearl farm/ is an essential building to help with prime crystal farming and as for the talent garden/ its another essential building needed to farm talents.
My humble abode:
With structures roughly around level 20