IdiotsOpposite
Boom, headshot.
I think you will find the description would read "no defensive bonuses from terrain", you will almost certainly still receive the negative -33% to defence on open terrain.
The alternative is not logical at all, and will all most certainly not be the case.
What do you mean it's not logical?
Or do you think that cavalrymen really WOULD fare better defensively in forests than in grasslands?
it's rather hard to do a cavalryman's job in forests...