The biggest problem with their plan was that it couldn’t be tested. When would they even have the chance? If they tried, it risked the lives of whoever was doing the testing- and would give away their plan. While someone might invisibly approach the wall, once they tried influencing the river to dig out under a wall it would be noticed no matter how well concealed other magic was.
Then the decision had to be made: was it good enough to take down the walls on the Bryrian side of the river? If so, it would be easy to send troops through the fallen walls. However, with the walls being more enchanted, even if the wards didn’t extend to the ground below the walls would be less prone to crumbling. If they committed several of their best mages to the attempt and they failed… they would have wasted all of their magic. Even if they retreated to recover and the losses to the army were minimal… the enemy would then know the plan. That was the problem- they could prepare for things because the enemy didn’t know what they would be doing. They didn’t have enough wizards to overpower defenses that were made to counter them.
With the difference in the level of enchantments on the walls, they could only be certain about dealing with the far side. There was little doubt they could bring down some of the walls there… but in that case, they had another problem. Their troops weren’t there. They couldn’t just get a squad there, either- it had to be a whole company of a hundred men. Anything less would be risky. Anything more would probably be missed on the battlefield.
Fortunately, that wasn’t something that needed to be solved by magic. It would be a slow and rather obvious process to transport an entire battalion of a thousand troops or more across the river… but just a portion of them could be managed much more quickly. Even with smaller boats, it would just take a handful of trips. As for where they would get the boats? They had tools available to construct them, and a few boats wouldn’t be noticed easily among the other work they were doing. They were still constructing siege ladders- while Major Erling was confident in the mage’s abilities, relying on just one specific course of action succeeding was foolhardy.
Then they would just carry them and plop them down in the river. As for why they couldn’t do it with more- besides it being more noticeable, having too many boats crossing the river at once was just inviting trouble.
Lucy was relieved the mages didn’t have to come up with a solution to that. Sometimes, regular manpower was the correct answer to a problem. Saving magic for where it was needed was the best choice.
Now the only difficult part was creating the spell to do what they wanted. It needed to primarily target below the wall, though it wouldn’t hurt if it also carried away more dirt. It had to both guide the water and empower it- if redirected, it would undermine the wall, maybe in only a few hours, but they couldn’t afford to wait that long. Especially not with enemy wizards watching and ready to kill them. The faster the better… but it also needed to be a stable enough spell that it wasn’t easily dispelled. If any standard unraveling magic worked, then it would almost certainly happen. They might get lucky and have all of the wizards on the ‘front’ walls, and maybe they wouldn’t notice… but they didn’t want the plan to depend on getting lucky.
Then there was the issue of having multiple wizards working together. Just one wouldn’t be able to output enough magical power in a short time, and if they had to defend themselves against attacks at the same time… they’d have a company away from the rest for nothing.
Fortunately the particular sort of spell they were going for would work in parallel for the most part- but that didn’t mean they didn’t have to work out how to keep people from trying to control the same water and/or dirt. If their spells tugged against each other, even if they were controlling in the same general direction they would waste precious magical reserves.
If the spell was a combined spell such that each mage had a specific part, it wouldn’t have that problem- but then any single one of the mages losing concentration would ruin the whole spell. It was also quite difficult to construct such a spell, though they did have. Sadly they didn’t have the reference book for group magic- only Douglas had seen that and read it, not thinking he might actually need it.
In short, they decided that having one generic version of the spell would be good enough, and they could coordinate their efforts, with whoever was going concentrating on one part of the river and bank.
As for who would actually be going… it was decided that Lucy would be one of them. It was an important job, and she was the only ‘senior mage’ besides Lundgren. He would be with the main force, because he was the most senior… and because Lucy would probably be more effective. The next most powerful was unfortunately Magnus, but it was decided he would stay with the main force as well- they didn’t think he would cause trouble, but Lucy didn’t want to make him feel important either. However, she would be taking Jonathan Herbert- the rather unassuming vospian mage who was similarly powerful. At the very least he had never been openly antagonistic, and he was quite competent. Isabel would be coming along as well- not as part of the wall destruction effort, but just as backup firepower. Since it was better to have more backups, they would also be bringing along Morgan Altair, an apprentice wizard whose only real fault was that he was young. Lucy judged his level of talent to be similar to Isabel- though with Isabel being a sorceress it was hard to make a direct comparison. It wasn’t that he was much younger than Lucy herself, either, but he hadn’t started studying magic in his early youth. He only had a few years of study before being called to war- whereas Lucy had a decade and a half.
That left the rest of the mages with the main force, and hopefully a few missing mages wouldn’t be noticed too early. If they were lucky, the enemy didn’t have an accurate count of their numbers.
Once the plan was set, all that had to be done was finishing the development of the spell… and while they couldn’t practice on the real thing, they could at least do some work downriver. There was a lot of riding away from the city involved with that testing, but it was worthwhile to iron out some issues they ran into- and real practice was important. Lucy just hoped eroding the river bank easily translated into walls collapsing.