I have a question about the fog of war. Why are areas that I know I have explored now re -covered by the fog of war?

Necromancer castles have a building called Cover of Darkness the reintroduces the fog of war around the castle for all other players at the beginning of each turn. There is also a map building that heroes can visit to cause the same effect.

I thought maybe enemies were closing in on me?

What is the "map building" you refer to?

This one. It works just like Cover of Darkness from Necromancer town, however whenever you want to use it, you have to visit it with a hero.

Thanks for helping