I remember Warhammer Siege - can't remember, but I think the General's Compendium may have some rules in, and then later on the Blood in the Badlands Campaign contained rules for sieges.
IIRC, there were some Daemon Siege Towers made for the Storm of Chaos campaign, but there were no rules. There was a battle report that had two stages, a Skirmish battle between empire state troops+militia versus the wall-climbers which led into forsaken, but I've forgotten their names (They had a special rule that aided climbing walls for units after) - which would open the gates if they succeeded.
The Khorne one had a great big battering ram, and the Tzeentch one was floating. The nurgle one I can't remember. There might have been a Slaanesh one.
If doing Bretons, an Immense Trebuchet might be an option.
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