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Regarding the towers in Heroes, the maximum damage I've seen them deal (this was about a month ago) was: the central tower dealt -1800 damage, and the side towers dealt around 1500 each. The attacking hero didn't have exceptionally high stats; I don't remember the exact numbers, but it was something like 7-8 Azure Dragons, 8-10 Red Dragons, etc. It was a Swamp Castle, by the way. I only encountered this level of damage once. There were also a couple of other maps where I reached the same point in the game, and the enemy was never able to capture my main castle. Consequently, experience kept accumulating. The last time, the enemy attacked with considerable forces, but the towers only dealt 300-350 damage. And what this depends on – I have no idea. But I doubt it's related to the duration of owning the castle...