From PCGamer UK
...a subtle but major change to the Total War formula: every unit now has dynamic, terrain-based line of sight, and no enemy unit is visible by default. No more steering your men towards a general magically marked out by a star on the battlefield.
"Each individual man is actually looking around him," Ferguson explains. "He can only see what he can see. As a result of that you get a much more claustrophobic effect when you're in a forest situation - and much less time to react.