I have some idea as to how this could work:
1. Players control separate parties on the map.
2. When one player is initiated in a battle, no other party on the map can interfere with that battle party. (Also create a safe distance zone around that party so no other parties can get close until the battle is finished, to keep from spawn killing someone with casualty-ridden party.)
3. If you want to join or oppose a player in a battle you either have to join their army or the opposing side.
4. Timer is the only problem, there will be no fast forward and no pause. If you want respite, go to a city or castle.
5. If one player is logged out, their party will disappear from the map entirely. If they log out within a city, and that city is conquered by the foe, they will simply be able to leave the city once logged back in.