Heroes 4 castle siege mechanics

After searching the internet, I cannot find the exact mechanics of castle siege in Heroes 4.

Browsing through some websites in English and Russian I have found that units on the towers get 100% increased attack and defense. Is this correct? Are there any other effects compared to the regular combat? Based on my experience I suspect that regular troops inside the castle walls (not on tower) also get certain bonuses? Is there a reference somewhere?