The Hightower Defenses for Euron's Attack on Oldtown

GRRM plans to open TWoW with two four major battles. One of them, the fan named "Battle of Blood" involves Euron vs. the Redwyne Fleet (which is previewed in the Forsaken). While it seems like Euron is planning a giant ritual sacrifice, we have no idea of the results. What we can do is look at his likely next step (no matter how grandiose or magical his attack is) which is the city of Oldtown.

Background

Oldtown is an ancient city (one of the oldest) and is one of the more defendable strongholds in the realm. It not only is home to the Citadel, but also House Hightower (I would argue Leyton is the most "famous" lord we have yet to meet in Westeros).

The Ironborn have been prodding at Oldtown already, when Sam and Co. arrive on the Cinnamon Wind, not only are they attacked en route, but they also encounter:

"My apologies," the captain said when his inspection was complete. "It grieves me that honest men must suffer such discourtesy, but sooner that than ironmen in Oldtown. Only a fortnight ago some of those bloody bastards captured a Tyroshi merchantman in the straits. They killed her crew, donned their clothes, and used the dyes they found to color their whiskers half a hundred colors. Once inside the walls they meant to set the port ablaze and open a gate from within whilst we fought the fire. Might have worked, but they ran afoul of the Lady of the Tower, and her oarsmaster has a Tyroshi wife. When he saw all the green and purple beards he hailed them in the tongue of Tyrosh, and not one of them had the words to hail him back."

Sam was aghast. "They cannot mean to raid Oldtown."

The captain of the Huntress gave him a curious look. "These are no mere reavers. The ironmen have always raided where they could. They would strike sudden from the sea, carry off some gold and girls, and sail away, but there were seldom more than one or two longships, and never more than half a dozen. Hundreds of their ships afflict us now, sailing out of the Shield Islands and some of the rocks around the Arbor. They have taken Stonecrab Cay, the Isle of Pigs, and the Mermaid's Palace, and there are other nests on Horseshoe Rock and Bastard's Cradle. Without Lord Redwyne's fleet, we lack the ships to come to grips with them." -AFFC, Samwell V

If interested: All Aboard!: The Journeys of the Cinnamon Wind

The Hightower Defenses

"What is Lord Hightower doing?" Sam blurted. "My father always said he was as wealthy as the Lannisters, and could command thrice as many swords as any of Highgarden's other bannermen."

"More, if he sweeps the cobblestones," the captain said, "but swords are no good against the ironmen, unless the men who wield them know how to walk on water."

"The Hightower must be doing something." -AFFC, Samwell V

Stronghold

The Hightower is one of the strongest seats in the realm, featuring a massive stepped tower with a beacon on top (higher than the 700 foot wall). Labyrinthine square fortress of black stone at the foundation.

The city itself is built in stone, with high stone walls as well:

Oldtown was built in stone, and all its streets were cobbled, down to the meanest alley.

Lord Leyton

Leyton Hightower has been atop the Hightower for a decade:

"To be sure. Lord Leyton's locked atop his tower with the Mad Maid, consulting books of spells. Might be he'll raise an army from the deeps. Or not.

If interested: The Man in the High Castle (he has a glass candle)

Ser Baelor Hightower

Leyton's eldest son (aka Baelor Breakwind from Oberyn/Elia's travels) has been building galleys:

Baelor's building galleys,

Ser Gunthor Hightower

We meet Gunthor when the Cinnamon Wind is inspected a second time on its way into Oldtown:

Gunthor has charge of the harbor

and:

They reached Oldtown on a cold damp morning, when the fog was so thick that the beacon of the Hightower was the only part of the city to be seen. A boom stretched across the harbor, linking two dozen rotted hulks. Just behind it stood a line of warships, anchored by three big dromonds and Lord Hightower's towering four-decked banner ship, the Honor of Oldtown. Once again the Cinnamon Wind had to submit to inspection. This time it was Lord Leyton's son Gunthor who came aboard, in a cloth-of-silver cloak and a suit of grey enameled scales. Ser Gunthor had studied at the Citadel for several years and spoke the Summer Tongue, so he and Qurulu Mo adjourned to the captain's cabin for a privy conference.

Ser Garth Hightower

Garth is training new recruits,

Ser Humfrey Hightower

Humfrey has been sent to Lys to hire sellsails:

and Humfrey's gone to Lys to hire sellsails.

Why? Because Lynesse (Jorah's ex) is the chief concubine of a merchant prince there:

If he can winkle a proper fleet out of his whore of a sister, we can start paying back the ironmen with some of their own coin.

Garlan/Willas Tyrell

After the Shield Islands fall, Garlan gathers men from across the Reach to help retake them:

"Highgarden shall answer," said Margaery Tyrell. "Willas has sent word to Leyton Hightower in Oldtown, so he can see to his own defenses. Garlan is gathering men to retake the isles. The best part of our power remains with my lord father, though. We must send word to him at Storm's End. At once."

and:

"How would the queen suggest they accomplish that, without sufficient ships?" asked Ser Loras. "Willas and Garlan can raise ten thousand men within a fortnight and twice that in a moon's turn, but they cannot walk on water, Your Grace."

and while the Shield Islands aren't close to Oldtown, it shows that not only has Leyton been prepping for a "bit" (I'd argue a LOT longer than that) but that men that would have potentially been there to defend Oldtown are off heading towards the Shields).

The Attack

While some might argue this is about somewhere else:

Then the towers by the sea, crumbling as the dark tide came sweeping over them, rising from the depths. -ADWD, Melisandre I

and:

"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

I strongly think it is about the Ironborn attack on Oldtown due to other quotes like:

The night they moved him, he had seen the moon floating on a black wine sea with a leering face that reminded him of Euron.

and:

The dreams were even worse the second time. He saw the longships of the Ironborn adrift and burning on a boiling blood-red sea.

But I don't think it requires even the Eldritch Apocalypse for this to happen. Mel could get this vision and it could just be about the Ironborn sacking the city for its wealth. Let's look at this quote from Obara:

"And how could you hope to hold Oldtown?"

"It will be enough to sack it. The wealth of Hightower—" -AFFC, The Captain of the Guards

and then go back to what Euron did with the Shields:

Victory is sweeter with a loved one by your side.”

“Your victories are hollow. You cannot hold the Shields.”

“Why should I want to hold them?” His brother’s smiling eye glittered in the lantern light, blue and bold and full of malice. “The Shields have served my purpose. I took them with one hand, and gave them away with the other. A great king is open-handed, brother. It is up to the new lords to hold them now. The glory of winning those rocks will be mine forever. When they are lost, the defeat will belong to the four fools who so eagerly accepted my gifts.” -The Forsaken

and what the Ironborn were initially trying to do in Oldtown:

Once inside the walls they meant to set the port ablaze and open a gate from within whilst we fought the fire

which would then result in even more chaos (which is Euron's goal):

Till then, the best we can do is guard the sound and wait for the bitch queen in King's Landing to let Lord Paxter off his leash."

The bitterness of the captain's final words shocked Sam as much as the things he said. If King's Landing loses Oldtown and the Arbor, the whole realm will fall to pieces, he thought as he watched the Huntress and her sisters moving off. -AFFC, Samwell V

TLDR: While I do think that Euron's attack on Oldtown will have a supernatural element to it, I think that even if one doesn't think the magic surrounding Euron will work/is legit they can reach the conclusion that Euron/the Ironborn will likely sack/take Oldtown.