Isengard can be attacked by only one army at a time, same as Hornburg. so the Ai cant place many stacks around it. u discourage it by holding full stack inside. try getting rid of some units if u want the epic fight. if u want the win let em have party there and send some nice assassins. i suggest epic fight, you gonna lose for sure but u gonna get plenty of money for losing settlement, and then again money for reconquering it.
the real problem about passive stacks is at good factions. several times i've seen stacks not attacking their enemies even when beeing sure they would annihilate them. waiting there for many many turns until the evils got reinforcements and wipe out everything. thats real odd, not agressive at all when necessary.