We need some good light source, that is relatively easy and cheap to keep on. That would produce a lot of light. Walking around dark village is too scary. And fireplaces every 5 tiles are no go.
For example, somekind of oil could be made. Like olive oil (or tar could be used). Lanterns would burn very long time with not much fuel. They'd be made from steel. Or iron. Could only be built on vclaims.