Upgrade your resource producing buildings and storage as soon as possible to level 5. Then, resume working on quests and the research requirements. Although you cannot build or upgrade two buildings simultaneously, you can easily do some research, building, and troop training at the same time. Always make sure research is ongoing because it requires a lot of time to complete. Silver coins can be used to purchase resource stacks. However, only buy them when they are really required to save coins for your heroes. Research Administration in the castle as soon as possible. It will allow you to have one more settlement in your kingdom, and thus increase your resource gain.