The dungeon was a tomb. It smelled of wet earth and centuries of neglect. But as I stood in the center of the cavernous room, holding a torch, I didn't see a prison. I saw a machine waiting to be turned on.
"My Lord," Captain Hareth said, holding a clay jar filled with a thick, black sludge. "This is the 'Black Blood' from the mines. We have barrels of it. If we burn it here, it will be warm."
"And we will all be dead by morning," I said, taking the jar.
I dipped a stick into the sludge and held it to the torch. It flared with a dirty, orange flame, sending a thick plume of greasy black smoke rising to the ceiling.
"Look at the smoke," I said, pointing to the ceiling. "That isn't just soot. That is poison. If we burn this openly, we choke the rabbits. If the rabbits die, we starve."
Giles shivered, wrapping his cloak tighter. "Then we cannot use the fuel? We will freeze."
"No," I said, tossing the burning stick into a corner. "We just have to change how we burn it."
I walked to the dusty stone wall and grabbed a chunk of white chalk. I drew a simple square.
"Imagine a blanket," I said to the confused soldiers. "If you put a hot stone on your chest, it might burn you. But if you put the hot stone under the blanket, the whole bed gets warm, right?"
Hareth nodded slowly. "Aye, My Lord."
"We are going to turn this floor into that blanket."
I drew a long line snaking back and forth across the floor of my drawing.
"We build a furnace for the Black Blood—a sealed stone box so the smoke can't escape into the room. Then, we connect clay pipes to the back of it. We bury those pipes just under the dirt floor."
I tapped the drawing.
"The fire burns in the box. The hot smoke travels through the pipes under our feet, heating the dirt, before it finally goes up the chimney and outside. The floor becomes a radiator. The room gets hot, but the air stays clean."
Giles's eyes widened. "Like... like a heated bathhouse floor?"
"Exactly," I smiled. "Radiant heating. Simple. Efficient."
"But the plants," Giles pointed out, looking at the sacks of potatoes and grass seed we had hauled down. "Even if it is warm, My Lord... plants eat sunlight. It is pitch black down here. They will starve."
"Correct," I said. "And that is where the snow comes in."
I pointed to the high ventilation shafts—narrow, square chimneys carved into the cliffside high above our heads. Currently, they were just black holes letting in a draft of freezing air.
"Hareth," I asked. "When you walk outside in the snow at noon, do you squint?"
"Aye," the Captain grunted. "The snow blinds you. It's brighter than the sky."
"That is called Albedo," I explained. "The snow acts like a giant mirror. It bounces ninety percent of the sunlight back up. Right now, we are wasting all that free light."
I turned to Tessa.
"I need every polished shield, every silver platter, every scrap of mirror the refugees have. We are going to place them at the top of those shafts outside."
I used my hands to mimic a bounce.
"We angle the mirrors to catch the light off the snow and shoot it straight down these shafts. We are going to pipe the sunlight down here just like we pipe the water."
Hareth looked skeptical. "But My Lord... if we open those shafts to let the light in, the cold air will pour down too. It's ten degrees below zero up there. The potatoes will freeze in an hour."
"A valid point," I noted. "Cold air is heavy. It sinks like water. If we leave the hole open, we die."
"We have no glass," Giles lamented. "It is too expensive."
"We don't need glass," I said. "We have the river."
I looked at the Captain. "Go to the frozen stream. Cut me slabs of ice. Not the white, bubbly stuff—I want the clear, black ice from the deep flow."
"Ice?" Hareth frowned. "To keep out the cold?"
"Think about it," I said. "It is freezing outside. Will a block of ice melt in freezing air?"
"No..."
"Exactly. It's a solid rock." I grinned. "But because it is clear, the light passes through it. We put the ice slab over the hole. It acts like a window. It stops the wind, but lets the sunbeam through."
I stepped back, looking at my team. I needed them to see the whole picture, not just the parts.
"Here is the system," I summarized, counting on my black-stained fingers.
"One: The sealed furnace burns the sludge. The pipes heat the floor. The dungeon gets warm." "Two: The mirrors upstairs catch the blinding light from the snow." "Three: The ice windows stop the wind but let the light beam down to the plants." "Four: The rabbits and potatoes grow in the warm, bright room. We eat the rabbits. The rabbits eat the grass. We use their manure to grow more grass."
I looked at them. "It is a circle. A living engine. As long as we feed the furnace, we survive the winter."
Silence hung in the damp air for a moment. Then, I saw it click in Hareth's eyes. He didn't see a dungeon anymore. He saw a bunker.
"Orders, My Lord?" Hareth asked, his voice snapping with newfound respect.
"Get the shovels," I ordered. "We have a floor to dig up."
By evening, the Keep was a hive of activity.
The refugees, usually listless and freezing, were moving with purpose. The promise of "hot floors" had motivated them faster than any whip.
I stood in the courtyard, watching Tessa and a group of children. They were polishing old bronze shields with sand until they gleamed like gold.
"Angle it forty-five degrees!" I called out.
Up on the snowy ridge above the dungeon vents, two knights were securing a slab of crystal-clear river ice over the opening. Beneath it, they positioned the polished shield.
Flash.
A beam of brilliant, white light suddenly cut through the gloom of the dungeon shaft, hitting the dirt floor below with the intensity of a spotlight.
"It works!" Giles shouted from the bottom of the shaft, his voice echoing up. "My Lord! It is bright enough to read by!"
I let out a breath I had been holding. The physics held up.
The Hypocaust furnace was already being bricked up by the blacksmiths. The clay pipes were laid. By midnight, the heat would start radiating through the soil.
"You turned a cave into a garden," Elara said, appearing beside me. She was wrapped in her heavy wool cloak, but her hood was down. She was watching the beam of light with wide eyes.
"I turned a cave into a battery," I corrected softly. "We are storing the sun and the oil to release it when we need it."
I looked at the invasive grass seeds in the sack. They were nasty, hardy little things that farmers in the south hated because they couldn't be killed.
Perfect, I thought.
"Elara," I said. "Tomorrow, you are in charge of the rabbits. If we do this right, in thirty days, we'll have meat."
"And if we do it wrong?" she asked.
"Then we eat the potatoes," I shrugged. "And hope they taste better than charcoal."