Not sure if Dominions_3 will have siege units or not... but I believe it will be possible to make at least some siege units via modding.
Siege units such as a Battering Ram or Siege Tower which allow for storming more quickly, catapults which can be used on the battlefield for long distance area attacks during a battle, and even design some siege units which can be used as counters to target and more easily destroy siege type units.
I believe most siege units should exist on the battlefield where opponents have an opportunity to destroy them.
Would love to hear ideas
.................. especially developer ideas.
[b]
EDIT: Just wanted to add I don't know the overall opinion about the mass-castle strategy currently in Dominions, but if designed correctly siege units might address this issue where gamers focus on building of castles more strategically instead of everywhere.