The medal requirements are shown on the mission select screen. For gold, you need to keep all cores and get a higher score. To get a higher score, build the minimum amount of towers and only when needed. The score is partially based on available resources and the sell value of towers at the end:
- Resources in the bank earn interest in story/campaign and challenge maps. Don't build a tower before you need it. That way you can keep resources drawing interest in the bank for as long as possible.
- The sell value of a tower is 75% of its cost. Therefore, towers reduce the score by 25% of what is spent on them. For example, a fully upgraded cannon tower costs 1,400 resources. It will sell for 1,050. An unnecessary Lvl 3 cannon tower reduces the final score by 350 points plus points for interest lost on 1,400.