What does the red x over Mills and Fields mean?

[agriba]

I'm assuming it means losing income, but those buildings are always in the most indefseable location so that doesn't make sense either.

Not seeing any explanation in game; please help.

[123]

It means you don't receive income from that building for that morning. So, you need to make sure to protect your investments.

[Life is Comedy]

it indicates wich economy buildings have been destroyed during the night, therefore not giving income. It is usefull when you cant see the whoole map during the night. To avoid, either protect thoose buildings better, either try building/uprgrading somewhere else first.