I think you have to defeat enough Isengard hosts they get desperate and disgorge one last big host. Defeat that and Isengard's defenses are weakened enough to directly attack the tower.
You probably also have to do the Fangorn quest, but I don't know for sure.
Edit: Isengard hosts don't appear until the war starts, so there's that, too.
Edit: On more careful reading it looks like you might have knocked the faction strength down enough pre-war by killing off caravans and such that it's actually possible to get the quest pre-war. Or you've found a bug.