Cereals are in fact species of grass; they're fast-growing and produce nutritious grain, but the human body is biologically-disadvantaged and we don't have four stomachs like a cow to be able to digest grass. Instead, we've had to apply our brains to the problem and invent technologies to aid our bodies. We need to physically grind wheat grain into flour and then use the transformative effect of fire in an oven to bake the flour into bread and release nutrients that our bodies can absorb. In this way the millstones in a windmill or waterwheel are like a technological extension of our own molar teeth, and the oven we use for baking bread or the pot we use for boiling rice is like an external pre-digestive system.

The primary problem is how you productively cultivate cereal crops in the first place – food surplus is the fundamental basis of any civilisation. If one person can feed 10 others who are not rooted to the fields and can specialise in other skills, then your society becomes more capable. Tools for working the soil like a plough and harrow could be scavenged, or created by repurposing steel items with a simple forge. But the crucial trick, one that evaded medieval farmers, is how to consistently maintain the fertility of your fields over the years. Without modern artificial fertilisers, you'll need to replenish nitrates in the soil by ploughing-in animal manure and cycling leguminous plants – peas, lentils, clover, alfalfa – with your cereal crops. Dissolving bones in acid will provide phosphates, and spreading crushed chalk or limestone will counter rising soil acidity.