I'm gonna push back on #3. What's going to attack fields of wheat? Dire deer? Were-gophers? Fiendish crows?1. Bread is a cheap, freely available staple food.
2. You need huge fields outside the walls to make lots of bread.
3. Huge fields outside the walls will inevitably be attacked and overrun.
4. No more bread.
As @Umbran points out, your typical monster is not looking to gobble up a bunch of wheat stalks. It wants to gobble the farmers. So that's where the defense would be focused. If you're near the edge of a monster-haunted wilderness, farmers will live in fortified clusters of buildings, and agriculture will be oriented toward the least labor-intensive crops (to offset the cost of protecting the farmers while they work the fields). The fields themselves are pretty safe.
In civilized areas, there won't be a lot of monsters to begin with, and agriculture will follow a more typical medieval model.