Defense Grid: The Awakening

You get a silver medal automatically if you get the gold medal, and since there's also an achievement for getting gold on all stages, you might as well go for that. A silver medal has the same requirements, except you only need a few hundred or thousand points less than for gold.

To get a gold medal, you have to finish the stage without letting a single core escape (enemies can grab them, but you have to kill them before they leave the map). You also have to get above a certain score designated in the stage select screen.

To get a higher score, conserve your resources. If you try to go for overkill, you won't save enough resources to get the score needed for a gold medal. Try to find the sweet spot where enemies can't get away with the cores, but it's still a bit of a challenge - get there and don't buy / upgrade anything.

Some people tell you to sell your towers right as you're killing the last enemy. Don't bother - the score will be the same. Your final score is a combination of 1x(tower value) and 1x(resources), so selling a tower for 100 resources means you lose 1x100 in tower values and gain 1x100 in resources... it's the same thing. Now, if you can sell the towers a few minutes before the stage ends, the extra resources will help your interest.

There are Youtube videos showing the best tower placements to get gold medals on each stage, but if you're familiar with tower defense games, you shouldn't need them for any of the stages with the possible exception of the finale.