Assume that the time period takes place at some point before cannons and other later siege equipment (medeival, antiquity, bronze age, etc.)
Stone mages can affect materials that follow the geological definition, which are “naturally occurring and coherent aggregates of one or more inorganic minerals“.
Important note is that it only affects compounds with a crystalline chemical structure, not amorphous ones such as obsidian and other glasses. It doesn’t affect organic minerals, or ones that are currently within a living creature (such as bone). So things such as Clay, Sand, Crystals, Glacier-ice, and other types of rocks ARE able to be moved, carved, and otherwise manipulated, while glasses, metals and other materials ARE NOT. An interesting case is dirt, which is only around 50% sand/ silt/ clay, and the rest of it is air, water, organic material, etc. Dirt can probably be controlled, but less efficiently than regular stone.
Magic is relatively rare in this world, but widespread enough that most organized armies would have multiple stone mages, and it really only takes one or two of them to easily knock over any wall built out of stone. To control stone in this magic system, the mage has to ‘resonate’ with the stone by playing some type of rhythm associated with it (similar to an instrument, except using their own body, i can explain this further if it helps answering). Different types of stone have slightly different rhythms that the mage needs to play to control it.
I have considered things such as wood, but the issue is that wood is flammable, and easy to throw a large boulder at using stone magic. This leaves basically two solutions that I’ve thought of:
- Find some type of material that stone mages can’t easily control, is cheap/ easy to source, and can be used to build large structures
OR - They don’t build wall at all. Similar to how castle walls fell out of use after the invention of cannons and mortars
Edit: also it’s easier to control rocks that are closer, compared to further away.