I definitly think there should be something like your disease idea so that there is more of an incentive to initiate a castle attack and I like the idea of a shoooting tower. Maybe you would have to get a unit to the base of it inside the castle and set it alight to stop it? The trouble with unlimited ammo is that the odds are fairly heavily in favour of the defenders anyway and it would take a very long time to run out of musket balls anyway.

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Fight first, ask questions later.