[ologm]

A stealthed unit that auto generates units can siege indefinitely.

Scenario:
Sneak stealthy unit that auto generates units, example a pan with meanads, into enemy province with castle.
Defeat province defense and patrolling forces in the province somehow.
The enemy is now almost unable to stop the siege.
The enemy can break siege from inside castle or send in new troops, but after defeating the sieging forces the liberating forces move into the castle, after which a new unit( meanad ) is generated in the now undefended province and the castle is under siege again.

Only way to stop it is with assasination spells or have a third party help you.