Cap army sizes. maybe 500? maybe 1000? pick a number, balance the spawn timers and such around that number. rather than having hugely increasing upkeep costs on bigger armies, just straight up cap the army sizes. Also, rather than having 10 guys with 500 man armies combine at the last second into a 5000 to attack, you literally have to attack 500 at a time, but the defender only gets 500 as well (unless you want to allow garrisons to have a bit bigger force). The attackers would besiege a fief, have some guys on the prowl for people trying to sneak support in, and have more armies around to continue the siege if the first army cant get it done, so if you REALLY want to defend a fief and you know there are 4 guys with 500 man armies coming, then you best send some of your own 500 man armies to intercept them (thus more field battles). It would still be possible to overwhelm with numbers, but it would take longer as you would have to attack over and over (which is how people are playing now anyways).