The final installment in my contribution to the enormous and probably mostly pointless library of worldbuilding advice on the internet. Today, we talk about the exciting things and why the internet is so bad at them.
You are not Brandon Sanderson. You cannot sell your world on the strength of a Brandon Sanderson-like magic system because Brandon Sanderson is better at being Brandon Sanderson than you are and has already cornered the market on being Brandon Sanderson. If you set out to make an original magic system that is exactly like what Brandon Sanderson would make, you have already failed, because that is not how originality works.
Now that the PSA is out of the way, we can talk about magic. Your job here is almost the opposite of what you do everywhere else. Whereas most worldbuilding is taking mostly familiar pieces of the existing world and trying to reassemble them in such a way as to exceed the limitations of a rote copy of reality, magic can do literally anything and you need to apply some limits to it in order to have any story at all. Those limits can come in the form of a hard set of rules that govern how magic interacts with the physical world, or they can come in the form of only being available to a small handful of people, none of whom have access to the stuff when it matters. If your protagonists have ready access to magic with undefined limits, then it’s difficult for the reader to have any idea what the stakes are, since at any time the party wizard might bust out a new spell that instantly solves whatever problem they’re currently facing. If they have allies with that kind of magic without having it themselves, it’s not clear what obstacles, if any, could not be solved by sending a runner back to Castle Wizardry to ask their friendly neighborhood wizard to wave the obstacle away. If magic lies entirely in the hands of mostly indifferent third parties, then it doesn’t need any defined limits at all, because you can keep magic from resolving the story instantly by having those third parties decline to get involved for personal reasons. It’s fine if Tom Bombadil could solve everything and he just doesn’t because it’s not his problem and he doesn’t really care.
Saruman and Gandalf have more defined powers and limits. Neither of them are especially hard limits, but Gandalf clearly specializes in fire and Saruman in beguiling, both of them are limited to spells that range from “party trick” to “mildly helpful,” and they both rely principally on their deep knowledge of the world from both long study and personal experience to get things done, to the point where Gandalf regularly draws a sword and personally fights orcs. The limits are never made extremely, explicitly clear, but the implications of their limitations are strong enough that when the hordes of Mordor show up on Pelennor Fields, we immediately understand that Gandalf cannot call down a meteor to smash them instantly, or conjure a legion of fiery minions to fight them, or even shoot fireballs from his staff as magical artillery. We know from the get-go that the best thing Gandalf can do in most of the battle is give heroic speeches, help organize the defense, and personally stab some orcs, rather than provide overt magical firepower.
All this to illustrate the point: The more openly allied to one side or another in the conflict, the more you need to limit the ability of magic itself. The Tolkien example above did this implicitly by making magic very covert, but you could also have magic be extremely overt but require limited resources or be dangerously unstable. You can give one or more of your characters reliable access to a sizable library of magical superpowers, so long as that list is fixed and doesn’t get added to without letting the reader know in advance, which is how Harry Potter does it: Provided they have their wands (they almost always do), Harry and friends have reliable and nearly-free access to a list of magical spells that constantly grows as the series goes on, but every power on that list is established as a tool in their arsenal before it solves any plots. Once Upon A Time introduces a new magical superpower on an almost episodic basis and its caster characters have an essentially arbitrarily large amount of spells in their library, but again, spells and magic items are established before they’re used to resolve the episode’s plot.
It’s important to think about more than just your plot when designing the limitations of your magic, however. If you establish that wizards can only cast one spell per day, getting their voodoo back each sunrise, that puts sharp limits on what your protagonist wizard(s) can accomplish for sure, but if wizardry is common, that means that most wizards are going to be using their daily spell for economic reasons if it’s at all profitable to do so. If a farmer can use his one spell to grow a crop from just planted to ready to harvest in a single day, he’ll do that, harvest manually, and grow several orders of magnitude more food than his real world counterparts. That ability would take a standard medieval peasant from an order of magnitude less productive than a modern farmer to an order of magnitude more productive than a modern farmer and will drastically reshape the economy. If you establish wizards as being able to set up permanent teleportation gates between two locations and also as working for (or becoming) kings or other powerful people who care about the well-being of this or that nation, then they’re going to set up permanent teleporters to other kingdoms’ capitals for diplomatic reasons, which will also drastically expedite trade.
Then there’s interpersonal applications of spells. If beguilers can mind control someone and are at all common, then kings and nobles are going to be extremely wary of letting strangers in to see them. Love potions are going to have all kinds of deeply messed up effects on dating and courtship, which will penetrate every level of society if they’re even slightly common, whether that penetration comes in the form of rumors and gossip about the new stranger in town being a wizard who’s here to literally charm their way into the mayor’s daughter’s pants, or if wizardry is so common that love spells are the roofie of your setting.
Every power you add to the standard arsenal of a wizard is going to have effects on the rest of the setting, whether wizards are Merlin-style rare and powerful advisers or make up 10%+ of the population. Be sure to think through what every spell you make available does not just for your protagonists, but for the setting as a whole. There are plenty of ways to limit this impact – a potentially transformative spell like the crop growth example could only be available to a tiny fraction of the population, like seventh sons of seventh sons, or you could attach a cost to it that’s steep for farmers but trivial for adventurers, like draining the nutrients from the ground and making the field fallow for over a full year, thus making the spell harmful in the long run for a farmer, but a wandering vagabond doesn’t need to care about soil conditions if he’s skipping town the next morning anyway.
There are three basic building blocks to the pre-gunpowder military: Infantry, cavalry, and archers (or technically “artillery,” which also covers things like slingers). Infantry are slow, tend to be armored with at minimum a shield and helmet, can be packed into tight formations, and can take the momentum right out of a cavalry charge and shred them in melee with more densely packed formations. Because they’re also really slow, archers can wear infantry down. Although infantry shields are, used properly, very good at intercepting arrows, even light casualties can cause an infantry formation to rout if there’s nothing they can do about it. It takes a good deal of grit and discipline to just steadily advance on an enemy, arrows constantly lodging themselves in your shield, and every ten or twenty seconds a gap opens up in the shield wall at just the wrong moment and one of your mates eats an arrow. Most infantry formations will bug out under these circumstances.
The accepted counter to archers was typically cavalry, because cavalry move quite fast and thus, unlike lumbering heavy infantry units, have relatively little time to ruminate on how they’re only one unlucky shot away from being the next fatality but still two hundred yards from being able to actually stab any of the bastards currently raining death on them. Archers need both hands for their bows and thus do not have shields, which puts them at a huge disadvantage in a melee, so once cavalry reach the archers, the archers would be nearly helpless against them, especially since they can’t volley while in melee, but instead have to rely on either drawing sidearms (daggers or short swords or the like) or else on trying to nock an arrow, aim it, and loose it at someone who only has to make a single quick stabbing motion to spill their guts on the ground. A skilled archer can load and loose an arrow very quickly, but not faster than a skilled swordsman can jab them in the kidneys.
You might be wondering why not just take some infantry and arm them with bows, which they can drop in favor of sword and shield if cavalry get close, and the answer is that you could use the same gear to equip a unit of infantry and a unit of archers, and swapping from bows to sword and shield and then forming up to meet a cavalry charge before said charge connects is not easy, so you risk losing your one-for-the-price-of-two formation anyway. That’s not to say that there couldn’t be an army in your setting that does this, and if they’re really well disciplined and well trained they might even be able to pull off the rapid switch from volley formation to tight packed infantry formation reliably, which would indeed give them a battlefield edge that would probably make them famous in the region, though it wouldn’t be a big enough advantage to make them unstoppable or anything.
So if your world is medieval fantasy (and it usually is), or if it’s iron age instead, then these are the standard mundane units you’ve got to work with: Infantry that beats cavalry, cavalry that beats archers, and archers that beat infantry. If your world is relatively mundane, you can probably just leave it at that, but you probably have combat wizards or ogre infantry or dragon riders or something else crazy, and it’s important to consider how they interact with these basic three units, as well as each other. Combat wizards of your blasty fireball sort are going to be either similar to archers or similar to cavalry depending on their range and radius.
A long range, wide radius blaster wizard is going to be a hyper powerful archer who can cause such devastation to infantry formations that even the kinds of disciplined and effective soldiers as made up things like the Roman legion, units that were ordinarily nearly impervious to morale damage from archers and quite good at coming out on top of infantry bloodbaths, will be sent fleeing for the hills because about a quarter of their cohort was just vaporized by someone two hundred yards away and there’s more where that came from in about five seconds, so if they don’t rout they will literally all die.
A short range, small radius blaster wizard is going to use his power right before connecting with the enemy to disrupt their formation. A cavalry formation that’s fairly dense with these sorts of wizards might be able to defeat an infantry block by smashing their dense formation right before the melee starts and then running down the survivors like they were routing. Once infantry are no longer densely packed, they lose their most decisive advantage against cavalry, while the cavalry still have a significant height advantage and are much faster, so wizards who can disrupt enemy formations with small radius blaster magic (or, for that matter, illusions or conjured thickets or whatever) can turn a cavalry unit from vulnerable to infantry to effective against infantry – and thus basically unstoppable for as long as the wizards’ magic holds out and isn’t somehow dispelled or warded against (if that’s even possible in your setting), at least until the unit gets winnowed down through combat, since even victorious units take some casualties.
Superheavy infantry like ogres might be less vulnerable to arrows if their hide is thick enough to absorb them as minor injuries rather than likely fatal wounds, and their massive advantage in size and strength would make them favorites to win almost any infantry vs. infantry fight, even taking into account that the smaller size of the enemy allows them to pack their formations much more densely, because they can plausibly wound an enemy straight through their shield. On the other hand, they can’t pack themselves nearly as densely as regular size infantry can, so cavalry would be more effective against them, especially during an initial lance charge. Even so, they wouldn’t be nearly as vulnerable to cavalry as archers, which means ogre superheavies are at worst only slightly vulnerable to any mundane unit and extremely effective against many of them. Any army with a large reserve of these would thus be extremely effective against a pure mundane army, but would still be vulnerable to superpowerful artillery units like the long range, wide radius blasty wizard given above. Additionally, ogres are big and eat a lot, which means every ogre you field is probably costing somewhere in the neighborhood of four times as much in terms of supply as a human would (depending on exactly how much bigger they are), so a human army may well be able to overwhelm an ogre army in sheer numbers.
Flying units can only be attacked by archers or other flying units. Units that are incorporeal but can still harm corporeal creatures are invincible unless they have some kind of weakness that can be exploited (destroyed or paralyzed by light, can be banished by a short incantation that can plausibly be taught to all the soldiers in the army, put out of commission every spring by seasonal allergies, whatever). Dragons are a terrifying culmination of flight, massive artillery power, and near invulnerability that makes them incredible game changers on the battlefield, to the point where an army without dragons would have to build their entire strategy around countering them to stand a chance.
It’s very possible that exactly zero of these examples are at all relevant to your world, but hopefully they’ve got your gears turning as to how whatever weird races, creatures, or magic powers do exist in your world, you have an idea of how they’ll change the way battles are fought.
There is more to war than winning individual battles, though, and another important consideration is supply and army movement. One of the biggest considerations in any real world military campaign from the pyramids to modern day is how to get your army from where it is to where it needs to be without starving to death or dying of dysentery. The majority of casualties in major wars was, until the last few centuries even, deaths of starvation or disease while on the march. That’s not to say that battles weren’t important, because the goal of a war is not to kill the enemy army down to the last man, but simply to defeat them in battle. Most defeated soldiers fled rather than getting killed, but defeated they were, and even if an army retained 95% of the forces it started with after losing a fight, it had still lost and would almost certainly get the same result if they had another go, if they can even find all their fleeing soldiers and get them in formation again. Armies that are “destroyed” have usually mostly thrown down their weapons to run away as fast as they can rather than being killed.
Nevertheless, when the day of battle comes, having more units on the field is good, so every man you lose to disease along the way is bad. This means that magical means of transporting large amounts of food more efficiently than with wagons, or especially of conjuring large amounts of food on the spot, are a significant advantage. Equally so are magical means of curing disease. Even minor injuries can be fatal if they become infected, and pre-modern medicine was mostly hopeless at dealing with infections, to the point where amputating an infected limb and then tying off the new wound with clean linen was often the best option available. Sometimes the resulting amputation wound would get infected anyway, or the patient would just die of shock. If both sides have access to food transportation/conjuration and healing magic in equal measure, then ultimately the effects will cancel out and on the actual day of battle things will go down much the same as in the real world (modulo whatever crazy fantasy units are on the field), but it will still change the look of the camp, in that soldiers will be lining up for the food conjuring rather than getting trail rations out of their own pack.
Magical means of army transportation can be a much bigger deal. Often one army’s goal will be to intercept another, and if the attacking army can just get around the defending army and capture a certain vital city, they will have won, whether the defending army could have defeated them in the field or not. The ability to march into a different dimension and then back out somewhere different in their home world would have a huge impact on such a situation. The ability to teleport directly to the enemy city would be massive, in that it would mean a world where attacking armies can form up and then drop themselves immediately onto a critical strategic location like the enemy capital before the defenders even know there’s a war on. If there’s no way to stop a wizard from dumping a raiding party straight into an enemy ruler’s throne room, they might end up in a MAD situation, where no ruler wants to declare war on any enemy with wizards because that will result in the rulers on both sides being killed by scry-and-die tactics.
As with the battlefield tactics, the examples of strategic and logistical differences I’ve given probably do not apply specifically to your setting, but hopefully they’ve gotten the wheels turning about what the fantasy elements in your world will change regarding military.
So that’s it. That’s the entirety of my rambling, unfocused filler content concerning worldbuilding. I could go on about this pretty much forever, but I do need to sleep at some point and I have projects I actually care about seeing through to completion that I really should be working on instead. Hopefully at least one or two people out there find some of this useful.