The "Tower(s) by the Sea" (Spoilers Extended)

Background

In this post I thought it would be fun to look at the "towers/tower" by the Sea that Melisandre sees in her vision in ADWD.

Visions danced before her, gold and scarlet, flickering, forming and melting and dissolving into one another, shapes strange and terrifying and seductive. She saw the eyeless faces again, staring out at her from sockets weeping blood. Then the towers by the sea, crumbling as the dark tide came sweeping over them, rising from the depths. Shadows in the shape of skulls, skulls that turned to mist, bodies locked together in lust, writhing and rolling and clawing. Through curtains of fire great winged shadows wheeled against a hard blue sky. -ADWD, Melisandre I

and:

"Some may." Could the skulls in her vision have signified this bridge? Somehow Melisandre did not think so. "If it comes, that attack will be no more than a diversion. I saw towers by the sea, submerged beneath a black and bloody tide. That is where the heaviest blow will fall." -ADWD, Melisandre I

Note: Melisandre is great at receiving visions in the flames, her fault is when she tries to interpret too far in order to seem more powerful

Eastwatch-by-the-Sea

With it being our first option presented to us, I think the odds of it being Eastwatch are quite low, but I also thing it is worth noting what Mel says here about things being "different but that also being the way with visions":

"Eastwatch?"

Was it? Melisandre had seen Eastwatch-by-the-Sea with King Stannis. That was where His Grace left Queen Selyse and their daughter Shireen when he assembled his knights for the march to Castle Black. The towers in her fire had been different, but that was oft the way with visions. "Yes. Eastwatch, my lord." -Melisandre I

Blackcrown and Three Towers

On the entrance to the Whispering Straight are these two castle on the north and south coast of the Honeywine. This is likely where the battle/blood ritual between Euron and the Redwyne Fleet will take place:

A deep blue dusk was falling as they entered Whispering Sound. Gilly stood beside the prow with the babe, gazing up at a castle on the cliffs. "Three Towers," Sam told her, "the seat of House Costayne." Etched against the evening stars with torchlight flickering from its windows, the castle made a splendid sight, but he was sad to see it. Their voyage was almost at its end.

"It's very tall," said Gilly. -AFFC, Samwell V

the wording seems to scream Euron to Me: (crumbling as the dark tide came sweeping over them, rising from the depths/I saw towers by the sea, submerged beneath a black and bloody tide).

If interested: The Sea/The Tide: Parallels in Jojen/Melisandre's Visions

Oldtown Itself

the mighty Hightower, with its great beacon, the tallest tower in all the known world. Truly, the Reach is a land for superlatives. -TWOIAF

and:

When first glimpsed in the pages of history, the Hightowers are already kings, ruling Oldtown from Battle Isle. The first "high tower," the chroniclers tell us, was made of wood and rose some fifty feet above the ancient fortress that was its foundation. Neither it, nor the taller timber towers that followed in the centuries to come, were meant to be a dwelling; they were purely beacon towers, built to light a path for trading ships up the fog-shrouded waters of Whispering Sound. The early Hightowers lived amidst the gloomy halls, vaults, and chambers of the strange stone below. It was only with the building of the fifth tower, the first to be made entirely of stone, that the Hightower became a seat worthy of a great house. That tower, we are told, rose two hundred feet above the harbor. -TWOIAF

Other Options

There are several other options that fit with much less likelihood:

• Starfall

Lady Ashara threw herself from a "tower by the sea" (body never found):

And they told how afterward Ned had carried Ser Arthur's sword back to the beautiful young sister who awaited him in a castle called Starfall on the shores of the Summer Sea. -AGOT, Catelyn II

and:

Or was it the grieving sister, the Lady Ashara? She threw herself into the sea, I'm told. -AGOT, Eddard XII

and:

But Ashara's daughter had been stillborn, and his fair lady had thrown herself from a tower soon after -ADWD, The Kingbreaker

• Pyke
• Storm's End

Of towers, there was but one, a colossal drum tower, windowless where it faced the sea, so large that it was granary and barracks and feast hall and lord's dwelling all in one, crowned by massive battlements that made it look from afar like a spiked fist atop an upthrust arm. -ACOK, Catelyn III

Issues/Problems

I think most of the other options Eastwatch/Storm's End/Slaver's Bay/etc comes with enough problems that I don't need to discuss them here. I think that the Ironborn attacking Blackcrown/Three Towers or Oldtown fits almost like a glove, except for one thing... Sam's story arc. I change my mind a lot on what exactly I think happens but if Sam's storyline in Oldtown is to go anywhere this probably is about the smaller castles during the battle of blood. That said if GRRM is trying to make this section of TWoW go absolutely crazy he could speed it up for an attack on Oldtown (Sam would then either have to flee, hide or become a prisoner).

TLDR: Mel tells Jon that Eastwatch-by-the-Sea is the "tower by the sea" that will be attacked by a "black tide". While she has seen something correctly, she likely has misinterpreted this with an attack by Euron on Oldtown (and/or the surrounding castles known as Blackcrown and Three Towers).