Ok new question
What is the lowest level magic that can be used en massed to change the rules?
- Shape Water cantrip can be very effective in winter campaigns, clearing snow & ice from roads, building igloos and ice houses, removing ice from firewood, creating ice-bridges over streams, etc
- Mold Earth cantrip can dig ditches, make berms, and repair roads.
- Create Bonfire cantrip produces a significant amount of heat without fuel for as long as your caster is awake.
- Firebolt & Produce Flame cantrips can't directly heat an area but can heat stones and iron pots
So let's assume 8 hours of travel in winter where the caster making the terrain less difficult than it should be. At some point the caster rests for 8 hours. Leaving 16 hours for camp activity.
So let's set up a camp. Use the first two cantrips to erect crude shelters that block the wind and help trap heat. One caster could make a 30x20 structure 10 ft high (well, 5ft deep + 5ft walls) in 5 minutes, assuming they had 5ft deep snow or dirt to work with. Each of those could hold around 60 troops or maybe 12 horses. So in an hour they could create shelter for around 720 troops or 144 animals (or some mix where each animal counts as a couple soldiers) .
As the caster finishes each set of walls, the troops would be stretching their tent canvas over the tops to be a roof as well as caring for pack animals.
The caster spends a few minutes erecting a mess hall and some amount of time providing the magical heat for food. Assume an hour of cooking before the first soldier eats. And then I'd probably guess 700 people could cycle through a mess line in about 2 hours. The same 3ish hours would apply to preparing breakfast as well. These times assume all infantry; animals wouldn't need to pass through a mess line.
Which gets us to 8 hours travel, 8 hours rest, 1 hour setting up camp structures and 6 hours of being an oven. That leaves the caster a whole hour to not be casting spells. Well, probably not. They likely spent part of that erecting a defensive barrier of earth or packed snow around the camp.
So, one 1st level caster with the right cantrips should be able to materially improve the winter campaign capabilities of several hundred infantry or a couple dozen cavalry. The worst road hazards are mitigated at least in part, they are well protected from the wind, aren't scavenging for as much firewood, dont have to do as much work to secure the camp, and get 2 hot meals a day (as long as they have food).
