So Arya called a council meeting, the topic being how to observe the sea for invasion, without putting their secrets at risk. They went back and forth, trying to figure out how to fly or keep something that flew tethered to the ground. How to keep someone up in the air well-equipped enough to stay there long enough to be of use. None of that worked; they couldn't see how it would. Then they started talking about building a tower. A tower would have to be 60 stories tall to be of any use. No one had ever heard of a structure that tall standing up to winds, and how could they keep it warm in winter? The stonekin came up with an idea, though: what if they built the tower on top of a mountain? What if they built it on Stonekin Mountain? Then it would only need to be about 30 stories tall. A really large base for the tower could be built, with steel anchors into the mountain itself. They could use steel beams bolted together to make the frame, and then build the stone tower around them. One of them said, "What if we build a shaft from above our furnace, right through the mountain, and use steel pipes for one of the corners, instead of beams. The pipes could funnel the exhaust from the furnace all the way up to the top, heating the tower."
They went round and round about whether the stone could stay solid that high, given the combined weight of the stone above it exerting that much pressure on it. Just the shaft alone would take a very long time to cut, but other stonekin said that cutting it would offer a chance to strike better veins of minerals on the way up. They discussed it back and forth, and slowly they came around to the idea. They called the meeting and were to return in two days after really thinking it through to vote.
Bran was contacted about this idea and asked for his input. It took him a day, but after talking to his maesters, he let them know he voted yes. Plus, he would send many workers and soldiers to help build the project. He said it should be called the Tower of Westeros. The next day, the vote was held, and with the promise of help from all over Westeros, they voted to build it.
First, they decided on the beam measurements, then on the pipe's size and weight. Once these were determined by structural need, they sent the measurements to the whole country. Foundries everywhere began producing needed parts, pipes, and beams. A system of grooves and ridges was devised to interlock the stones together. Each block had to be carved to match the one under it. Gendry devised a strap system that could be attached to the steel beams, wrapping through and around each set of stones to help lock them in place. His forge went to work mass-producing these straps.
Slowly, the cut rose upwards from above the stonekin forge. Platforms had been built to keep stone from falling on the huge forges that were making the beams and pipes. As the cut rose pipes were joined and anchored into the walls, and all of it had to be kept perfectly straight up through the mountain, so that it would come out exactly where the opening was being prepared above. Teams from the local villages were trained by the stonekin and paid by Bran and Arya. They took to the work and became accomplished stone masons. Gone were the animosities that Arya's becoming Warden caused. The entire country was working together to see this structure built and used to protect them all.
Thomas went to work to create a spyeye large enough to allow clear sighting 100 miles away. They tried method after method until they came up with the proper number of lenses, spaced exactly far enough apart to produce a clear image at that distance.
Vehicle and ship loads came every day from all over Westeros, and a large gantry system was built to lift all of that product the mountain swiftly.
Three months later, the cut through the mountain was done, and the pipe rose to catch up with the beams already in place. The stones were laid to catch up to the other three walls that were already much higher. The brackets that locked the walls to the beams and the cross bracing were used for the carpenters to build platforms and landings for stairs. The stairs rose higher and higher.
It was fall, and the weather was getting colder, so the pipes connected to the forges below began to heat up. The stonekin were amazed by the suction created by the flue formed by the pipes going upwards. Their fires burned hotter and cleaner than at any time in their memories.
The first real test of their work came in the form of a massive storm from the east. The winds blew at 40 miles per hour, and the rain came in sheets. They covered the top opening with tarps and hunkered down to see if the 25-story structure would stand.
It went off without a hitch; there were some issues with water runoff, but those were resolved later. The Tower of Westeros stood without a flinch.