Regarding Units defending against a Siege ...

[Norfleet]

Originally posted by Teraswaerto:
Do all units that are not mindless and do not have a castle defense bonus defend equally well? Do things like size, strength or moral have an effect?

I think that this is the case. Ability to fly may be a bonus in castle sieges, but that could just be flavor text.

Units that are besieging a castle, however, are worth varying amounts depending on size, strength, siege bonus, and ability to fly. Mindlessness is not a drawback when sieging the castle.