The Mirror of Galadriel

[AntiXZer01]

I was just reading the book and came across this paragraph in the Chapter "The Mirror of Galdariel"

"Then there was a pause, and after it many swift scenes followed that Frodo in some way knew to be parts of a great history in which he had become involved. The mist cleared and he saw a sight which hehad never seen before but knew at once: the Sea. Darkness fell. The sea rose and raged in a great storm. Then he saw against the sun, sinking blood-red into a wrack of clouds, the black outline of a tall ship with torn sails riding up out of the west. Then a wide river flowing through a populous city. Then a wide fortress with seven towers. And then again a ship with black sails, but now it was morning again, and the water rippled in the light, and a banner bearing the emblem of a white tree shone in the sun. A smoke as of fire and battle arose, and again the sun went down ina burning red that faded into a grey mist; and into the mist a small ship passed away, twinkling with lights. It vanished and Frodo sighed and prepared to draw away." (Tolkien, 379)

I was wondering what it was refering to. At first I thought it was refering to Númenor and it's history. I thought the tall ship with torn sails may have been Elendil's ship comming from Númenor to Middle-Earth after Númenor's downfall and the rage of the seas. The wide river through the populous city is probably refering to Osgiliath of Gondor but it could be another city in Arnor? The main misunderstanding is with the next vision, a wide fortress with seven towers? I can't think of any one city or fortress with seven towers. Could it have been Annúminas!? After reading it over the only other possibility is that it is Minas Tirith, but Minas Tirith of Gondor did not exactlly have seven towers, it had seven tiers rather than towers. The next few visions I perceive as being the battle of the Pelennor Fields and the arrival of Aragorn on the black ships of the Corsairs of Umbar.
Im not sure about any of this and so I was wondering what some of my fellow Lord of the Rings Gurus think it is speaking of. After all Galadriel says herself the mirror shows things that were, things that are, and things that have not yet come to pass. What seen in the mirror, sometimes even the wisest cannot tell. Yet im sure someone else has an idea?