Castle upgrades take priority or maintaining your resource level buildings at all the same levels?

Well the castle upgrade probably takes priority. But you will need to earn a large amount of resources to let you keep leveling it up so try no to let the rss production buildings fall too far behind. Depending what other buildings you want to upgrade (eg the troop training grounds) you will need a lot of certain resources at different times.

If you are very active and can make a lot of resources from attacking others and by completing hourly challenges and other events for the resource rewards then the RSS production is less important. But if you are less active then good production rates are very important.