Importing stone is also very expensive so its not illogical to imagine that in an area surrounded by trees there would be a large ammount of wood used in ALL construction be it fortress or outhouse.
Also, why is the politeness of a war declaration roleplay which suggests the ENEMY is lesser than its authors such a matter of contention. It is almost certainly a propaganda roleplay.
And fortresses are also very expensive. Nobles pay good money to buy them. And doesn't it say something about you 'signing the last block' when you make fortifications? If it is in blocks, it is highly likely stone.
Propaganda is words. But, unless he is hallucinating, sights can only be changed to a degree, even if you are looking at them with biased eyes.
