There are heavy woods, but I don't know if they block line of sight. I suspect they do. I'm certain hills block LOS.
Hook
The heavy woods block line of sight... in fact, the calculation tries to be precise...
For example:
A (on a hill, 50m)
B (at 0m) ||||||||||||||||||| forest|||||||||||| C (at 0m)
B can't see C.. but it's possible A can see C (just a geometric condition)
Other example
A B _______________ C
A is an Infantry unit.. A doesn't see C
A is a Cavalery Unit and B an Infantery unit.. A sees C
So, it's not always easy to know if an unit can see another one..
That said, HW calculates the threat with the visibility, but also the sound (at short range).
These mechanisms are hidden to the user.
JMM