Reach 400 citizens without building crop fields, orchards, or pastures.
This is a simple achievement once you resist the temptation to build crop fields, orchards, and pastures. Starting the game on hard will be an easy reminder of this, as you will not start with any animals or seeds.
You can still trade for food from merchants if you worry about food variety. Just be sure to have enough food production to handle the lack of farms, orchards and pastures.
Maps with lots of water will be beneficial, as some well-placed fishing docks can produce good amounts of food.