The goal is to collect 50 batches (orbs) from the Treasure Trove (fountain), located somewhere on the map before your opponent.
You can get money by killing monsters (bats, mummies, scarabs, snakes and faraos). You can spend money to guns, turrets, collectors (harvesters), bombs, rails and rail-droids. You can buy those things at your Base Camp. While you're shooting, you can't pick up any coins. Just stop shooting and they will come to you.
To collect those 50 batches, you make a track (rails) to the Treasure Trove, and place rail-droids (with the R key) to collect the batches.