The first argument began because Tyrion's hands were cold.
Not dangerously. Not enough to make a maester panic or a servant scream for blankets. But cold in the way Mordred had learned to hate—deep, lingering, the sort that settled into him and seemed reluctant to leave once it got there. At seven, Tyrion had become skilled at pretending discomfort was beneath comment if comment might cost him his work. He could sit through accounts with pale fingers and stubborn dignity. He could hide fatigue under sharp questions and turn coughs into irritation rather than weakness if given the chance.
He was doing exactly that in Joanna's sea gallery on the morning the idea took hold.
The room was bright enough, the winter light silver over the western water. A good fire burned in the hearth. Braziers had been placed at careful intervals by servants who knew better than to leave Tyrion at the mercy of draft and stone. Yet Casterly Rock was still a fortress. Rich, ancient, magnificent, and full of old cold bones no amount of velvet entirely conquered.
Tyrion sat at the long account table with a wax tablet in one hand and a stack of port tallies near his elbow, wrapped in crimson wool and trying with all his little strength of pride to look like a boy who had not been chilled through by the room itself.
Mordred looked up from her own ledger and noticed at once.
"Your hands are stiff," she said.
Tyrion tucked them under his arms with immediate offense. "They are not."
"They are."
"They are merely cold."
"Yes."
"That is not the same thing."
"It is when it's you."
Joanna, sewing by the window with the easy half-attention mothers perfected after years of pretending not to monitor everyone they loved, glanced up. "How bad?"
Tyrion looked personally betrayed. "Mother."
"Answer."
He sighed with all the world-weariness of a tiny old man born into the wrong body. "Only a little."
Mordred looked at the fire. Then at the braziers. Then at the stone floor. Then at the walls.
The answer came whole.
Not as memory named from elsewhere. Never that. She did not think in those terms aloud, not even in her own head if she could help it. The old life stayed buried. But principles remained principles whether whispered by ghosts or born fresh in the mind. Warm the room itself. Warm the structure. Move the heat through the stone instead of losing it to air.
Her eyes narrowed.
Joanna saw it immediately. "What are you thinking?"
Mordred stood so quickly the chair scraped back across the floor.
Tyrion groaned. "Oh no."
Mordred ignored him. She paced once. Twice. Then stopped and pointed at the floor.
"Lannister hypocausts," she said.
Both Joanna and Tyrion stared.
"Lannister what?" Tyrion asked.
"Heat beneath the floors. Through channels. Through the walls. Drawn and vented properly. Stop trying to warm one part of a room and expecting the rest to behave."
The gallery went still.
Mordred's mind had already run ahead. Fire chambers below. Raised flooring over stone piers. Warm air channels. Wall flues. Controlled draft. Proper smoke draw through new chimneys rather than relying on every room to choke around a single open hearth like some overdecorated peasant hall.
Joanna lowered her sewing into her lap. "You can do that."
It was not even quite a question. Just Joanna recognizing the tone.
"Yes."
Tyrion, suspicious by nature and smarter than most men in the Rock already, studied her face. "Will it work?"
Mordred looked at him. Truly looked. At the too-thin wrists hidden in wool. At the pale fingers. At the little body always trying harder than it had any right to need to.
"Yes," she said. "It will."
Tyrion frowned. "That sounds expensive."
Joanna laughed softly.
Mordred's mouth curved. "That sounds like Father's problem."
Tywin heard of it before noon.
Of course he did.
Nothing expensive, structural, or useful had ever breathed inside the Rock without reaching him in some form. By the time Mordred entered his solar that afternoon, he had already seen a steward's note, one message from the chamberlain asking whether masons should be called, and some condensed version of "Lady Mordred means to set fire under the floors and calls it progress."
He did not look up at once.
"What are you rebuilding now?" he asked.
Mordred shut the door behind her. "The Rock."
That made him lift his eyes.
Good.
She came straight to the desk and spread one rough charcoal plan over the carved wood between them. A sketched room cut in section. Channels below. Raised floor. Heat moving through controlled space. Wall shafts. New chimney draw.
"It's not madness," she said before he could speak. "It's better heating."
Tywin sat back slightly.
"The room itself should hold warmth," Mordred continued. "Not just the air near a single fire. I want fire chambers below selected rooms, raised floors over stone supports, heat channeled through the understructure, flues carried through the walls, and smoke drawn properly through improved chimneys instead of bleeding into every inhabited room like we're pretending soot is noble."
Tywin said nothing.
That was often a good sign.
Mordred pressed on. "Start with Tyrion's chambers. Mother's solar. The account room. The smaller library. Interior rooms first where the gain is highest. Then expand once we know the exact draw and labor cost."
Tywin looked at the plans. Then at her.
"Why now?" he asked.
Mordred exhaled once through her nose. "Because Tyrion is still fighting winter room by room like an enemy army, and I'm tired of old stone behaving as though wealth and intelligence should stop at tapestries."
There.
That landed.
Tywin's face changed by almost nothing, but she knew him. The reason mattered. Tyrion mattered.
"How extensive?" he asked.
"Enough to matter."
"How difficult?"
"Very."
"How certain are you?"
Mordred held his gaze. "Certain enough to ask for architects."
That sharpened him.
Not because he objected. Because asking was not her first instinct. It meant she had already reached the point where the thing exceeded one person's oversight, and she knew it.
"You can't manage it alone," he said.
There was no judgment in it. Only fact.
"Not at full scale," she answered. "I can design it. Direct it. Correct it. But I want builders and architects with enough sense to translate principle into stone without me standing over every cut."
Tywin's gaze flicked, just once, to the lower curve of her middle. Not yet much. Not enough for strangers. Enough for family. Enough for him.
"And you will not overextend yourself proving your point to craftsmen," he said.
Mordred lifted a brow. "That sounds like an order."
"It is."
He set aside the charcoal plan and reached for a blank sheet. "I'll send for them. The best from Lannisport and Oldtown. Masons, builders, chimney specialists if any claim that title honestly, and men clever enough not to mistake unfamiliar work for impossible work."
Mordred allowed herself the smallest smile. "Good."
Tywin took up his pen. "You will produce room priority, fuel requirements, estimated labor disruption, and safety concerns."
"I know."
"You will also write down every element you consider non-negotiable so that when you're not present, no one claims confusion."
That was excellent advice, which made it annoying.
"Yes, Father."
Tywin paused only once more before beginning to write. "If this succeeds, I want the same system adapted in stages elsewhere."
Mordred blinked. "Elsewhere?"
"The capital. The royal nursery. Any family chambers where my grandson spends winter." He did not say Joffrey's name because he did not have to. "And eventually wherever Tyrion chooses to work most often."
That touched her more than she wanted to admit in front of him.
"Then it will succeed," she said.
Tywin inclined his head once. "Make sure it does."
The architects arrived within the week, and none of them knew what a Lannister hypocaust was until Mordred explained it.
That was as it should be.
She called them into one of the lower planning halls with floor diagrams, wall sections, airflow sketches, furnace layouts, and enough charcoal notes to make the table look like a campaign map for war against cold itself.
There were five of them in total.
Two Lannisport builders who had handled rich merchant houses and knew the local stone. One mason from the west strong enough in practical work to be useful even if he looked at the plans like they had personally offended his grandfather. One old chimney specialist from Oldtown who took one look at the venting diagrams and immediately asked the first intelligent question in the room. And one younger structural architect who had enough imagination to be dangerous if not supervised.
Mordred approved of him least. That meant he was probably necessary.
Joanna came too, not because she needed to be there, but because she knew better than to leave a room full of male craftsmen entirely alone with her pregnant daughter's temper. Tyrion sat wrapped near the side wall under warm coverings and two extra braziers, indignantly pleased to be present for a meeting concerning his comfort. Tyland came because where there was danger, noise, and drawing on a large table, there Tyland would somehow be. Mors, too large and too likely to body-check an architect into a wall by accident, had been banished to the yard with Jaime and a practice shield.
A mercy for everyone.
Mordred began without flourish.
"These," she said, laying a hand over the first room section, "are Lannister hypocausts."
The men looked down.
"Raised floors over stone supports. Heat chambers below. Controlled channels. Warmth moving beneath and up through the room instead of dying in front of a fire and leaving the rest of the chamber to rot in damp."
The old chimney man leaned closer. "And the smoke?"
"Not in the room," Mordred said flatly. "In the walls. Drawn upward through Lannister chimneys designed for proper pull."
That got their attention.
Now they were in familiar territory enough to start understanding how strange the familiar could become when pushed intelligently.
The Oldtown man tapped one sketch. "These shafts are too narrow here if the room above is occupied often."
Mordred nodded at once. "Then widen the draw and split the upper vent."
His brows rose.
Good. Let him learn quickly that disagreement did not mean ignorance.
The younger architect frowned at the floor supports. "If the spacing shifts under heavy furniture, won't you lose even warming?"
"Yes," Mordred said. "Which is why furniture placement gets considered in the room layout for once instead of after."
The western mason grunted. "That's not how great houses build."
"No," Mordred replied. "That's why great houses shiver under banners and call it dignity."
Joanna smiled faintly over her folded hands.
Tyrion, from his warm chair, looked thrilled.
For nearly two hours the discussion stayed useful.
Heat movement. Stone tolerances. Furnace access below. Ash removal. Air flow control. Chimney draw in a cliff-built structure where sea wind changed pressure unexpectedly. Which rooms to do first. Which to avoid until later. How to keep labor disruption from turning the Rock into a permanent construction pit.
Then one of the Lannisport men—broad, capable, and just foolish enough to mistake her visible fatigue for weakness of mind—said, "My lady may underestimate how difficult it is to alter old noble floors on this scale."
The room chilled.
Mordred turned her head very slowly.
"No," she said. "You underestimate how little patience I have for men who say 'difficult' when they mean 'I have not yet thought hard enough.'"
Tyland giggled.
Tyrion actually sighed with satisfaction.
The builder reddened.
The older chimney specialist cleared his throat and pointed at the wall flue sketch. "The principle is sound."
Joanna's eyes flicked toward Mordred just in time to see the first sign of trouble.
Mordred's hand tightened on the edge of the table.
It was not dramatic. Not enough for the men at once. But Joanna knew her daughter's face, and she rose immediately.
"Mordred."
"I'm fine."
She was not fine.
The room smelled of hot braziers, lamp oil, men, mortar dust, charcoal, old damp stone, and too many bodies thinking too hard in one space. The nausea hit not gently but in a sharp rolling wave that made the floor tilt by half a degree and her stomach turn with murderous conviction.
She straightened, put one hand over her mouth, and walked out before anyone could be stupid enough to offer help in the wrong tone.
Tyland, delighted by all departures that looked dramatic, half rose from his seat until Joanna fixed him with a look that sat him back down immediately.
Tyrion watched the door with worried eyes.
Tywin entered from the corridor not thirty seconds later.
He had clearly been informed the meeting had reached the stage where craftsmen and his daughter were breathing the same air too long.
He took in the room in one sweep:
the plans the architects Joanna standing Tyrion alert Tyland alert in the wrong way Mordred absent
"Explain," he said.
No one chose the wrong answer.
By the time Mordred returned several minutes later, pale but composed, cool water on her wrists and all visible traces of weakness hidden except from family, the room had transformed.
Two of the braziers were gone. The windows had been cracked just enough for fresh air without draft. The architects stood instead of lounging. Extra scribes had appeared. Tywin occupied the head of the table and the old chimney specialist had already begun reorganizing the flue sketches into cleaner groups by room priority.
Mordred stopped in the doorway.
Tywin looked at her once. "You will review and direct. You will not stand for another hour proving your intelligence to workmen."
One of the builders stared fixedly at the table as if eye contact might get him killed.
Mordred folded her arms. "I was gone five minutes."
"That was long enough," Tywin replied.
There it was. Concern in his language, hard and controlled and entirely unmistakable if one loved him enough to hear it.
Tyland looked between them with bright fascination.
Tyrion, seated wrapped in warmth and already fully on his father's side in this conflict because cold rooms mattered more than adult pride, said, "You do look pale."
Mordred glared. "I could still throw you into the sea."
"No," Joanna said gently. "You couldn't."
"No," Mordred admitted. "I couldn't."
Tywin pointed at the chair beside Tyrion. "Sit."
She sat.
Because she was not stupid. Because her father was right. Because the architects would now work harder under his eye than they had under hers and there was no sense wasting fury proving it.
Tyland climbed down from his bench and leaned against her knee. Tyrion looked from her to the plans and then, after a long serious moment, asked, "Will it be warm everywhere?"
Mordred turned to him.
All the irritation. The interruption. The nausea. The bruised pride of being ordered to sit.
None of it mattered more than that question.
"Yes," she said.
Tyrion nodded once with solemn royal gravity, as though granting approval to a great public works project.
Tywin saw.
Joanna saw Tywin see.
And all at once the room remembered why it existed.
Construction began in Tyrion's chambers first.
Not because the Rock had only one fragile person in it, but because Tyrion was the clearest answer to the question of why improvement could no longer be deferred. His rooms. His solar. The account room he used most often. The smaller library where he liked to work in the afternoons when the light was best. Then Joanna's solar because she spent enough hours there with ledgers, letters, and children to justify sense.
The upheaval was appalling.
Floors were lifted. Stone cut. Furnace chambers hollowed below. New shafts carved into old walls. Existing chimneys modified and, where necessary, entirely rebuilt into what the household quickly began calling Lannister chimneys because no other term suited them better. They drew cleanly, more efficiently, and with less of the old smoke-stink noble houses tolerated out of habit and stupidity.
Mors loved the construction.
At five he had become a broad little menace with a body already too substantial for comfort and the instincts of a siege ram in miniature. He adored hammers, shouted orders no one had given him authority to issue, and declared the furnace rooms "excellent" after one look because they involved fire, stone, and men sweating.
Joffrey, visiting from the capital, approved no less.
The prince stood at the edge of one open floor section in black and crimson and pronounced the underfloor channels "dragon tunnels," which Mors accepted instantly as correct because it sounded dangerous.
Tyland liked the scaffolds better.
At three, all speed and bright danger, he moved through half-built spaces like mischief given bones. Twice he had to be retrieved from places no son of a prince and lioness should have physically been able to fit. Once he hid inside a partially finished flue shaft and nearly caused one apprentice to throw down his tools in fright.
Tyrion, wrapped and carefully kept away from dust, watched all of this from a padded chair at a safe distance and took notes.
Of course he did.
"Joffrey is trying to climb the furnace stair," he said one morning without looking up from his wax tablet.
Betha, mending nearby, sighed. "Again?"
"Again."
Mordred, reviewing revised chimney draw notes with the Oldtown specialist, did not even turn her head. "Mors is with him?"
"Yes."
"Then they'll either succeed or make enough noise to be stopped."
Tyland appeared beside Tyrion's chair at that exact moment as if conjured by chaos itself. "I got higher."
Tyrion looked at him flatly. "You're three."
Tyland smiled beautifully. "Exactly."
That answer told Mordred everything she needed to know about the future and none of it was peaceful.
The first rooms were completed before the worst of winter set in.
That mattered because once the Rock's true cold came in off the sea and deep through the stone, every fault in a heating system would reveal itself immediately. Mordred had insisted on testing before that. Good sense. Better arrogance. The furnace teams ran low and steady. Warm air moved under the raised flooring channels. Heat climbed through the walls. The Lannister chimneys drew cleanly.
The first evening Tyrion stepped into his own completed chamber under the new system, he stopped in the middle of the floor and simply stood there.
The room was warm.
Not patch-warm. Not hearth-warm in one corner and miserable elsewhere.
Warm.
The stone itself held it. The air did not bite. The bed curtains did not need to trap life by sheer fabric desperation. The writing table by the window could actually be used in winter without gloves. The room no longer felt like a battle against his body.
Tyrion looked down at the floor. Then at the walls. Then at Mordred.
"It's warm," he said.
It was such a small sentence. So stupid, to anyone else.
To them, it was victory.
Joanna's hand flew to her mouth.
Tyland sat down on the floor immediately because if the floor was now important, then obviously he had to feel it first. Mors dropped beside him and declared, "Good floor." Joffrey, not to be excluded from any royal or practical triumph, sat too and struck the boards with both palms to test them as if they might answer his authority.
"They're going to treat it like a game," Cersei said dryly from the doorway, one hand resting on Joffrey's shoulder.
"Good," Robert replied from behind her. "Means it works."
Tywin stood farther back.
Of course he did. Not close to the sentiment. Never intruding on the center of it. But present. Watching.
His gaze stayed on Tyrion.
The boy had crossed to the writing table by the window now and put both hands on it. Not shivering. Not pulling his sleeves down over stiff fingers. Not already tired from fighting cold before he even began working.
Tywin said only, "Better."
Tyrion looked over.
"Yes," he said.
That one word between them carried more naked love than half the court's songs ever managed.
Mordred saw it. Joanna did too. She glanced toward her husband and smiled in that quiet private way that said she understood exactly what he was and how hard he loved.
Tywin ignored the smile as if that could undo being seen.
That night, after the rooms had emptied and the children had finally been dragged away from testing whether warm floors improved wrestling or chasing or rolling, Mordred stood alone a while in Tyrion's chamber.
One hand rested lightly over the small swell of her middle.
Not yet publicly obvious. Not hidden from family. The child there—Elenei, perhaps, or perhaps another path first—felt very near and very far both. Her body still reminded her daily that she was not mistress of all things merely because she wished to be. The sickness had eased some. The fatigue came in waves instead. Work still called. Children still climbed things. Ships still needed refining. Joffrey and Mors still battered at one another with horrifying joy. Tyland still moved like a prince-shaped problem. Tyrion still pushed too far and drew concern from Tywin in tones that sounded like command because Tywin knew no other honest language for fear.
And yet.
The room was warm.
The Rock had changed.
Not by confession. Not by naming any dead world or lost history. Not by speaking of origins no one would understand and no one ever would. The knowledge stayed where it belonged—with her, buried and silent and taken one day to the grave.
What the world got instead was Lannister hypocausts. Lannister chimneys. Warm stone. Better air. Rooms made kinder for one brilliant fragile boy who mattered too much to be left at the mercy of old drafts and older habits.
Mordred smiled into the quiet.
Yes.
That was enough.
More than enough.