Build #165 adds area penalty to production.

The area penalty is based on the average area covered by your cities so that you have 100% production capacity only if your total land area divided by number of cities is 40 tiles or more.

If you have less than 100% capacity, all areas that you can reclaim by simply attacking them and killing whatever is there, will light up with a red crosshatch pattern. Destroying neighbouring enemy cities will give the same benefit but are obviously a lot harder to knock down, so they're not highlighted.