"I see…," Lord Oliver said.

But water seemed to spread the fire, not quench it. With each new dousing, the flames leapt higher. The men stepped back, confused. In the end, they watched helplessly as the trebuchet burned before them. In a few moments, it was a mass of charred, smoking timbers.

"By God, Edward and Saint George," Oliver said.

Johnston gave a small bow, smiled.

"You have twice the range and an arrow that alights itself - how?"

"The powder is ground fine and so explodes more fiercely. The arrows are filled with oil, sulfur and quicklime, mixed with tow. Touching any water makes them catch fire - here it's the dampness of the grass. That is why I have a basin of sand, should the slightest bit of the mixture be upon my fingers and start to burn from the moisture of my hands. It is a most delicate weapon, my Lord, and delicate to handle."

He turned to the third basin, near Marek.

"Now, my Lord," Johnston said, picking up a wooden stick, "I pray you observe what follows." He dipped the stick into the third bowl, coating the tip with the oily, foul-smelling mixture. He held the stick in the air. "As you see, there is no change. And there shall be no change for hours, or days, until

…" With the theatricality of a magician, he splashed the stick with a small cup of water.

The stick made a hissing sound, began to smoke, and then burst into flames as the Professor held it. The flame was a hot-orange color.

"Ah," Oliver said, sighing with pleasure. "I must have a quantity of this. How many men do you require to grind and make your substance?"

"My Lord, twenty will do. Fifty is better."

"You shall have fifty, or more as you will," Oliver said, rubbing his hands. "How quickly can you make it?"

"The preparation is not lengthy, my Lord," Johnston said, "but it cannot be done in haste, for it is dangerous work. And once made, the substance is a hazard within your castle, for Arnaut is certain to attack you with flaming devices."

Oliver snorted. "I care nothing for that, Magister. Make it now, and I shall put it to use this very night."

Back in the arsenal, Marek watched as Johnston arranged the soldiers in rows of ten, with a grinding bowl in front of each man. Johnston walked down the rows, pausing now and again to give instructions. The soldiers were grumbling about what they called "kitchen work," but Johnston told them that these were, in his words, the herbs of war.

It was several minutes later when the Professor came over to sit in the corner with him. Watching the soldiers work, Marek said, "Did Doniger give you that speech, about how we can't change history?"

"Yes. Why?"

"It seems like we're giving Oliver a lot of help to defend his castle against Arnaut. Those arrows are going to force Arnaut to push his siege engines back - too far back to be effective. No siege engines, no assault on the fortress. And Arnaut won't play a waiting game. His men want quick scores - all the free companies do. If they can't take a castle right away, they move on."

"Yes, that's true… ."

"But according to history, this castle falls to Arnaut."

"Yes," Johnston said. "But not because of a siege. Because a traitor lets Arnaut's men in."

"I've been thinking about that, too," Marek said. "It doesn't make sense. There are too many gates in this castle to open. How could a traitor possibly do it? I don't think he could."

Johnston smiled. "You think we might be helping Oliver keep his castle, and so we're changing history."

"Well. I'm just wondering."

Marek was thinking that whether or not a castle fell was actually a very significant event, in terms of the future. The history of the Hundred Years War could be seen as a series of key sieges and captures. For instance, a few years from now, brigands would capture the town of Moins, at the mouth of the Seine. In itself, a minor conquest - but it would give them control of the Seine, allowing them to capture castles all the way back to Paris itself. Then there was the matter of who lived and who died. Because more often than not, when a castle fell, its inhabitants were massacred. There were several hundred people inside La Roque. If they all survived, their thousands of descendants could easily make a different future.

"We may never know," Johnston said. "How many hours have we got left?"

Marek looked at his bracelet. The counter said 05:50:29. He bit his lip. He had forgotten that the clock was ticking. When he had last looked, there were almost nine hours; there had seemed to be plenty of time. Six hours didn't sound quite so good.

"Not quite six hours," Marek said.

"And Kate has the marker?"

"Yes."

"And where is she?"

"She went to find the passage." Marek was thinking that it was now late afternoon; if she found the passage, she could easily make her way inside the castle in two or three hours.

"Where did she go to find the passage?"

"The green chapel."

Johnston sighed. "Is that where Marcel's key said that it was?"

"Yes."

"And she went alone?"

"Yes."

Johnston shook his head. "No one goes there."

"Why?"

"Supposedly, the green chapel is guarded by an insane knight. They say his true love died there and that he lost his mind with grief. He's imprisoned his wife's sister in a nearby castle, and now he kills anybody who comes near the castle, or the chapel."

"Do you think all that's true?" Marek said.

Johnston shrugged. "No one knows," he said. "Because no one has ever come back alive."

Her eyes squeezed tightly shut, Kate waited for the ax to fall. The knight above her was snorting and grunting, his breath coming faster, more and more excited before he delivered the killing blow-

Then he was silent.

She felt the foot in the middle of her back twist.

He was looking around.

The ax thunked down on the block, inches from her face. But he was resting it, leaning on it while he looked at something behind him. He started grunting again, and now he sounded angry.

Kate tried to see what he was looking at, but the flat blade of the ax blocked her view.

She heard footsteps behind her.

There was someone else here.

The ax was raised again, but now the foot came off her back. Hastily, she rolled off the block and turned to see Chris standing a few yards away, holding the sword that she had dropped.

"Chris!"

Chris smiled through clenched teeth. She could see he was terrified. He kept his eyes on the green knight. With a growl, the knight spun, his ax hissing as he swung it. Chris held up his sword to parry. Sparks flew from clanging metal. The men circled each other. The knight swung again, and Chris ducked, stumbled backward, and got hastily to his feet again as the ax thunked into the grass. Kate fumbled in her pouch and found the gas cylinder. This foreign object from another time seemed absurdly small and light now, but it was all they had.

"Chris!"

Standing behind the green knight, she held up the cylinder, so he could see it. He nodded vaguely, continuing to dodge and back away. She saw he was tiring fast, losing ground, the green knight advancing on him.

Kate had no choice: she ran forward, leapt into the air, and landed on the green knight's back. He grunted in surprise at the weight. She clung to him, brought the canister around to the front of his helmet, and fired gas through the slit. The knight coughed and shivered. She squeezed again, and the knight began to stagger. She dropped back to the ground.

She said, "Do it!"

Chris was on one knee, gasping. The green knight was still on his feet, but weaving. Chris came slowly forward and stabbed the sword into the knight's side, between the armor plates. He gave a roar of fury and fell onto his back.