I'm pretty sure I missed this topic being discussed, so I would kindly ask if there is any info on a feature supporting multiplayer both on the campaign map and on the field per-say. It might actually be simpler than at a first glance: the players would engage within a map, especially made for multiplayer - or a section of the original map, or the whole map depending on the preferences and number of players - and simply engage in actions with one-another just as in singleplayer, but of course with much more options regarding diplomacy. This system would be crucial in multiplayer, giving each character the ability to send messengers containing any kind of message to any of the other characters.
More options should be implemented within such a system such as the ability to respond, intercept, kill the messenger etc. - allowing for a far larger range of interaction possibilities. The fact that these are between real players would make everything even a little 'easier' on the system, hence truces and alliances would be kept by the player themselves, allowing for them to be broken at will.
Two main problems would arise in such a multiplayer system though:
1. The overall time of a game.
2. The discrepancy between real-time fighting and the campaign-map time.
These could be quite easily overcome by simply twisting firstly a few starting status options and secondly some time-passing settings:
1. To decrease the time needed for a game to complete (which in single-player could last forever) each game should have customizable settings consisting starting options - similar to the Total War Series - where, within a consensus regarding a money frame, each player is able to invest within their belongings: be it army (troops), defense (castles and fortresses), economy (villages and trade) etc.
2. An overall goal to be achieved - realistic within the thought-of time period needed for a game to finish - such as the first to have a number of castles, the first to reach a certain wealth, the first to conquer a certain kingdom (be it NPC or player, who would initially have more wealth than all other players combined in order to stimulate alliances) or even a time limit to the game.
3. The real-time battles vs. campaign time: a simple fix would be simply slowing down the time passing within the campaign map and increasing the time passing on field. The effect would not be slower travel necessarily on the campaign map, but a longer period needed to reach the battle itself: for example there is a battle going on between two characters. One of them sends for help from an ally (real players). This ally can choose to rush to the battle and as they rush to it the battle of course continues between the two players normally. Once the ally has reached the battle, they would enter the battle field, but with an increased range/distance from the main battle. They would have to march all the way in oder to help. Of course, large maps would be an important aspect here, which I don't know how realistic would be. In this case a time limit (up to a few minutes) until one is able to actually join the battle would be thinkable. Within this time the character could take care of something else, eg. send out orders through messengers.
A simpler way would be the ability for the ally to send troops by themselves to help out. These troops would be put under the command of the one who needs support upon arrival and then return automatically when the battle is over.
4. Multiple fights at the same time: this could cause a problem. What if one player is engaged within a fight and has their castle attacked at the same time. Simple: the attacked partition would be just as is in the current system led by AI. However, the player should be able to implement a certain algorithm or strategy regarding reactions upon being attacked. For example one could have to option to play as defensive as possible, as in, NPCs run away in the castles and try to engage as little in hand to hand, or extremely offensive, as in, cavalry would go around the fortress, through the back, and attack while the archers and infantry stay inside.
5. Roles. Meaning of course one can choose to start as a nomad, as a barbarian, thief or lord etc.
Of course, if the main character dies within a battle, they are out of the game - alternative options are thinkable.
RECAP - it's simper than it seems:
How to implement multiplayer campaign games with real-time battlefields?
1. Each player has a money limit to choose their starting conditions.
2. Large range of overall goals (similar to Heroes of Might and Magic) to choose from.
3. Smaller or maps made especially for multiplayer.
4. Algorithmic programing (if...then statements) of AI troops.
5. Time overall equal during campaign and battles (slower for campaign, faster for battles in the sense of a day passing).
6. Adjusting of distance upon joining a battle.
It's not perfect, but definitely better than nothing.
if you need multiplayer that badly go and try war of the roses and stuff.
