I have observed, that sometimes Mordor Battletowers seem to be immune to the Akallabeth song of the Rivendell revelers. The visual and sound effects are still displayed, but the specific towers do not take any damage from the ability.
I have not yet found any rule that determines, which towers are immune and which take damage and only observed this, when I played against an AI Mordor. It seems, however, that a tower, that doesn't take any damage from one cast of the Akallabeth also receives none from any subsequent casts of the ability and is in fact completely immune to this ability.
