In plenty of fantasy words, there's a wizard who can drop fireballs on large swathes of combatants. There usually isn't a detailed explanation of how a medieval military force would deal with what is effectively precision targeted artillery.
How would the formations, tactics, and equipment of pre 14th-century change to deal with massive fireballs and poison gas?
For the purposes of this question, we'll assume these attacks can also destroy stone walls with a few hits.
This would have the same affect that cannons did in the real world. Slow, solid moving formations would vanish. Fast moving charges, especially cavalry would be highly valuable to quickly close the gap with the enemy before the aoe can wipe them out.
Armies would stop advancing directly on one another and instead would take advantage of shelter like woods and trenches/foxholes to shield themselves.
Fortresses would either be abandoned or adjusted. With the rise of the cannon, defenses started to be made out of sand or dirt. These materials could compress unlike stone and better withstand the shock of attack.
Eventually warfare would start to look like modern warfare, where everything from nukes to bombers force us to fight spread out, seek shelter that can protect us from bombs, either by hiding exactly where the troops are or physically protecting from debris, and fighting for air or magic supremacy. I can easily see magicians sniping each other or sending in assassins prior to an engagement to get magical supremacy. Equipment would change from metal armor, which would burn people alive and keep them too slow into lighter, more fire resistant materials.
As for poison gas, even in modern warfare that hasn't worked out well. The vagaries of the wind can send the gas back on your own troops and its hard to advance when you lay out poison gas in front of your advance. Troops could invent magical gas masks, fall back from their position until the gas passes, tunnel down to let the gas flow over, or use magical winds to disperse the gas or turn it back on their enemies.
One thing to consider is that you are describing an arms race. In an environment where magic is this common you should expect counter-measures to be created as well.
For example castle walls might have anti-magic enchantments upon them that protect both the wall and the men upon it from spells.
You might have wizards in your army using countermagic or defensive spells to protect the soldiers.
Equipment may well adapt as well, scatter a few shields along the shield wall that absorb fireballs and suddenly the threat is abated.
Answer to this will really depend on a few things:
The answer to these questions will ultimately determine tactics...wizards of this extent tend to favor the undisciplined forces more than well disciplined ones.
Undisciplined is a reference to skirmishers or barbarian type tactics where the unit fights individually or in a spread out area. Barbarian type warriors don't work well with others...in tight formations their attacks tend to interfere with each other as much as anything, so they tend to fight as scattered groups and put emphasis on individual heroism. Skirmishers also fight in loose formations, firing or throwing their weapons and then falling back. This tactic would be exceedingly useful as a method of disabling enemy wizards (range of wizards comes in here, if archers can get close, fire their weapon in the direction of wizards, then fall back before melee troops can engage them...you have an effective method of disabling wizards). A fireball would likely be deadly to the few caught in the fireball, but the loose formation prevents a total loss scenario that tight formations would experience (unless you wizard can machine gun fireballs with little regard to the number cast in a day)
Defence in depth also becomes more valuable, spreading defences deep instead of one layer. 3 walls instead of 1 giant wall. Destruction of the first wall is inevitable, but falling back to the second wall and defending as the enemy comes through the first wall is a great tactic. One thing of note...fireball range is usually less than a longbow or similar archer. Depending on your wizards range, the ability to send fireballs flying at a castle wall may endanger the poorly armoured wizard and put him in the range of archers. And another note...opposing wizards on castle walls can just as easily send fireballs back at attacking wizards. Defending wizards here have a great advantage...they can spot the attack wizards and attempt to disable them far easier than the attackers can spot them, they have a castle wall to hide behind, and if it's a layered defence, they can always fall back and defend from the second wall (or third, or keep, or fortress, etc...) as well. Remember defending wizards can be more dangerous than attacking ones.
Protracted sieges would be far less frequent. One of the larger time wastes in any medieval campaign is putting together the tools required (catapults, seige towers, rams, etc...) and the slow whittling of a castles defences. Since wizards could work as siege weapons to disable walls, the tactic of hiding behind walls with a far lesser force and forcing a protracted siege while reinforcements arrive is no longer a viable solution. Actually has the effect of making a rampaging army far more effective as the need for long sieges is gone
Disciplined troops is a little different as the size and strength of the fireball comes into play. Instead of relying on spread distances to reduce the effectiveness of an area effect attack, they are going to try to use each other to resist the attack. Heavy mail and armour, with thick padding underneath to reduce the effect of a fireball is most likely, along with a large shield. These units rely on each other to protect themselves, not just themselves...the front rows holding shields forwards with back rows holding them above. You don't give much information on how strong these fireballs are...so it's hard to tell if this is an effective attack.
Cavalry generally fall into two categories...fast and light cavalry will use their speed and agility to try to avoid and dodge incoming attacks, while heavy cavalry rely on their status as 'tanks' on the battlefield. Need more details on fireballs here to determine their effectiveness...though I would assume fireballs would have a pretty devastating effect on heavy cavalry. That said the best counter to skirmishers is generally cavalry as they can easily ride through loose formations and cause havoc.
Horse archers might be exceedingly effective...riding up, firing an arrow towards enemy wizards, then retreating while reloading to repeat. Cantabrian circles would ensure a stream of arrows flying at a wizard (or group of wizards) while maintain a pretty wide spread making area of effect attacks far less useful.
Gives an interesting loop... Skirmishers counter wizard, cavalry counter skirmishers, tight formations (spears) and heavy cavalry counter cavalry, wizards counter tight formations and heavy cavalry.
Edit:
Missed the poison gas section...
Poison gas is a hard tactic to use effectively and weather variations can make it as brutal on yourself as it is on them. It's easier to use in a defensive sense as the enemy has to advance towards you to attack and through the poison gasses to get there. Attacking with gas has the bad tendency to take down your own troops as they advance. Not sure what effects this could have on battles, tight formations on attack are obviously at the disadvantage here.
It basically boils down to how common these things are. They could be common or rare but known.
Rare but known means that such powers are wielded just by few of the most powerful masters of magic (such as high level PCs in RPGs), and perhaps lesser magi could have less potent forms of this, able to kill few men at once, but not to disperse a well-disciplined unit of 100 men. Then the difference wouldn't be as great, but elite armies (or ordinary soldiers when enemy is known to have some masters of magic) would be trained to disperse (and perhaps fall down, to mitigate destruction done by explosions) quickly, and then to form a shield wall again when enemy soldiers approach close enough that enemy is unlikely to throw more explosive fireballs to the area. This is the norm in most low- to mid-magic fantasy worlds.
When fireball-hurlers are really common (every army has enough of them to blast any army of similar strength grouped together into nothing in minutes), then the tactics must be different. Now soldiers would be trained to fight dispersed in small groups and only the elite would be trained to form a shield wall when no enemy mage can be seen.
This is true for attacks where scattering is the only defense. If poison gas is common, ordinary soldiers (elite if it's uncommon but known) would have some kind of primitive gas mask - usually a mask with some herbs believed/known to counteract the poison. If this is really common, some armies could develop real gas masks like in the first World War.
If this is a good siege weapon, the effect would be similar to effect of cannons. Google something on 17th century fortresses for some idea, or ask another, more detailed question.
