Posts from Random morality, luck, and abilities.
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To put it this way – the random element in the siege is fixed, but you can change the order of creatures, move to different cells, and perform other actions. Repeating the sequence of actions will yield the same result.
In the battle, the random element works based on frames; it won't be possible to influence morale, luck, or abilities (with standard randomness), but you can affect other actions by changing the animation speed (e.g., catapult shots).