The tower of Barad-dūr falls, as do the Towers of the Teeth and the walls adjoining the Black Gate. As for the Gate itself--it shivers to pieces, and falls into the gaping pit that has opened to swallow up all of the signs of the might of Mordor.
But the true fall of Mordor takes place away from walls erected by sentient beings, there where Orodruin tears itself to pieces. The mountain is caught in a case of indigestion of cosmic proportions, forced to swallow the foulness formed in its fires an age and a half past.
And at its feet huddle the two refugees, those guilty of bringing that abomination back to Its source. Balefully, the fires released from within the mountain creep toward them, seeking to soothe its distress somewhat with their relative innocence.
But in the end they are plucked away, and Aulė is there to calm the agony felt by the earth, reassuring it that never will it again be so abused as to bring forth such evil. However, one last fiery stone and tendril of smoke and ash is flung after them, a reminder that the earth itself resented the torment it had known under Saurons tyranny, and could resent those who caused it distress--before at last it relaxed and slept under its true Masters hand.