When playing Puzzles and Empires farming stuff is a major part of the game and doing it well will save time and resources. Food and Iron can be gotten from producing building with a steady income. The amount from building wont be enough though as your going to need a lot more.
Moving forward in the story and replaying mission is a great way to earn resources. You should be careful though as the cost to play a stage will go up the further you get. This makes it not so cost effective once the price is 4 energy and not 3 at first. Pushing forward will get you to a point eventually that will be worth the extra points, but until then you might want to farm on the stages that cost less and get you more resources. Stages with 3 and 4 waves will produce the same amount of goods and resources so playing the ones with 3 waves will save time and speed up gathering resources.
Quest will become available at a certain level and are a great way to earn resources like food and iron at an increases rate. Make sure to check often and beat the levels that aren’t too hard. They may cost more to enter, but the amount of goods earned will make up for it.
Raids are one of the fastest ways to get food and iron. Some opponents will have tons of loot with little effort in getting it. There are 2 ways to go about raids. The first is to constantly re roll with food to get an opponent that can be beat. This will more than likely reduce the amount of food gathered into the negatives making it cost food rather than winning it. The second way is to ignore your score and flee from other players that you know that you can’t beat. After fleeing you should get someone that can be beat or close to it. This can put food on the positive side when it comes to winnings.
Titans are another ways to get stuff and should be fought at least one turn to be included in the winning when it’s defeated. The stronger the Titan the better rewards and longer it will stick around giving you a chance to participate.
For more info check out some Empires and Puzzles Tips and Tricks.