Fields as map control element? Sounds overall interesting, but then you need to make sure that it's possible to create defensive options for your farms. But overall this seems to be a nice idea and gives your militia men a point. Either use women who are defenseless for harvesting fast, or use citizens to have a defense and harvest slower. However, you'd need the option to create defensive positions earlier in neutral territories - like palisades and wooden towers and maybe houses to protect you