The routine of the days had become something predictable without becoming repetitive. Saran was at the enrollment documents before the horse lines had run their morning feed, catching two more settler families that Khulgen’s deputy had missed in the previous week’s count.

Khulgen was at the supply tallies with Mahmud’s new cross-reference format in use, each tributary submission across two ledger rows instead of one.

The relay drills on the open ground north of Sarai each morning, and the sound of them reached the workshop district the same way the depot sounds did, as evidence in the air that the doctrine was working.

The friction with Orda’s formation was exactly as Torghul had said it would. In each combined exercise the White Horde’s relay riders moved through the new signal accurately and a beat behind Torghul’s riders, who had been doing it for months.

It was the expected pace of an armed body absorbing doctrine from outside its own tradition. The difference narrowed with each drill. Torghul logged it without commentary.

Siban was not in the camp. He had been in the tributary settlements south of Sarai for days, and no report had come north yet.

Batu had not expected one. The work he’d been given took time, and the winter had it to give.

Borte-Qol had left three days before, his household arrangements made and the channel retired. The promise Batu had made in the first winter of it had been honored.

When the engineers arrived, they came in a group of eleven. Persian stone specialists and Chinese engineers from the former Jin territories, organized through the eastern contacts on the Ayas relay.

Khulgen logged them the same morning they arrived. He came to Batu the following afternoon with the name of the man whose field experience matched most directly what the spring campaign would face.

The man’s name was Zhao. He had been in Mongol service since the final years of the Jin campaign, first under the eastern tumens’ siege engineering command and then through a succession of assignments as the campaign moved and the cities requiring work changed character.

He came to the meeting without ceremony, sat across the table from Batu, and said nothing until he had taken in what there was to take in. "Me and my men can set up a siege."

He continued. "We’ve done the crude and precise work at ten cities in the eastern campaign."

He paused, then went on, still watching Batu.

"But you are also asking us to set up the field for your army to move?"

Batu looked at him. "Tell me how you can deal with river crossings."

"We can build pontoon platforms where the ice wasn’t reliable. Timber float sections supported by rope on both banks, where the current allows it."

He considered the question behind the question. "I heard about the Oka River. A crossing the width of the main channel, firm bottom on both sides, takes a day and a half. Two days if the current’s running hard."

Batu nodded once. "If the materials are pre-staged."

"A day. Less if we’ve done the survey beforehand."

"The Rus campaign runs on rivers."

Batu said. "Every city of importance sits on one, and the relief routes between principalities use them to move. An army that can cross a river in a day arrives somewhere the defenders weren’t expecting it."

Zhao sat with that for a moment, forming the picture from what he’d been given.

"What does it take to bring a river crossing down," Batu said.

"Less time than building one."

He didn’t hesitate. "Fire at the support points if it’s timber. Undermining at the footings if the water level allows access. A bridge that takes a day to build comes down in a few hours if the work is done at the right points."

Batu let that confirm before he answered.

"And in that way we can stop a relief force from being capable to set up a crossing."

"If we reach it first," Zhao said. "Yes."

He was reading Batu differently now. Something in his eyes had changed, the way it changed when he understood what kind of problem he was actually looking at.

"You and the rest moves with the vanguard. Always behind the lead screen."

He touched the table once, lightly, as he spoke. "The engineer corps, we will call it. If there is a river my army needs to cross, the corps prepare it beforehand. If there is a river an enemy force needs to cross, the corps will have it removed. I do not want your services only for siege tasks."

Zhao absorbed this. It was a different perspective than anything he had worked inside before, and he didn’t pretend otherwise.

"We’d need to know the bridge structure in advance. Timber frame or stone pier, the standard material. It changes the method."

"You’ll have it."

He didn’t ask where from. He noted it as a fact about the organization he had joined.

"One more thing," Batu said. "The fortification preparation for the city assault."

Zhao gave a short nod, listening closely. "It’s the work that kept the main force alive in the eastern campaign," he said. "The earthwork screen, the graded lane to the fortification walls. Without it you lose men in the open before the assault even reaches the city walls."

Batu simply nodded, "Every Rus city is timber and earth above stone footings. The construction burns." Batu looked at the table. "The corps task is to reach the city before the assault force. It observes the defenses and finds where construction weak points. When the assault happens, it already knows how to demolish the walls."

"We’d need the city’s layout," Zhao said.

"You’ll have it," Batu said again.

The meeting closed where it needed to close. The corps moved separately, together with the vanguard in the march, ahead of the main body at every river and every fortified position.

It reported to Batu for operations. The crossing work existed in both directions, building for the army and denying to the enemy. The fortification preparation happened before the main body arrived.

The corps didn’t fight battles. It decided where battles happened and on whose terms.

Zhao understood it. He left the way he’d arrived, without ceremony, with more to do than when he came in.

Khulgen arrived before the afternoon light was done with a brief report he delivered at the pace appropriate to something that wasn’t an emergency but couldn’t wait either.

The craftsman from the Ferghana road had moved on his own and come through the Ayas relay directly to Sarai, without the spring review he’d been expected to want. He was in the camp’s eastern section with his materials and one assistant.

"Has he been settled," Batu said.

"Yes," Khulgen said. "He asked when you’d be available."

"Tomorrow morning," Batu said.

Khulgen made the note and left.

Outside, the camp had its evening sounds, the horse lines and the settlement fires, the army areas settling toward their night rotations.

The engineer corps was in its billet east of the main sections. The fire weapon craftsman was two sections over from that.

The spring campaign would start from a different position than the winter had found it, and there was still winter left.