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Interesting. I've also been exploring the siege mechanics a little and have developed a strategy to defeat a besieging army in the field, whereby winning the siege defense may be unlikely due to a combo of both the siege numbers game and a lack of decent siege AI.
It does work on the principle that you should have a sizable militia garrisoned in your castle though, for when the enemy begins it's siege.
If you arrive before the besieging army does, empty the castle of any decent units and take them into your army, leaving only the castle's militia to defend, which may be as many as 250 militia depending. If you don't have the room to take them, assign a follower, give them the units, create an army and have them join.
It matters not if the enemy has already begun the siege when you arrive and any units you want are trapped in the settlement, it just means the enemy will take longer and incur more casualies taking it, and also gives you more time to gather your forces.
Remain close by with your army and let the attacking army do it's thing. Keep a close eye on your militia troop numbers during the siege and when that number is reduced to a handful of defenders, as little as 5, attack with your army. In some cases i've found that this tactic is more than enough to win the day and save your castle without losing quality troops to wonky siege AI, instead giving yourself a fighting chance to command them in the field where they perform at their best, ahem! Castle militia will recover over time.
I also don't feel like this is in any way an abuse of mechanics but infact a perfectly sound strategy, weakening the enemy on throw away units (militia) then engaging them with your best units, ideally of better quality and organized for a real battle.
Sure it's still a numbers game, but there exists a certain numbers sized window to pull this off, and feels good doing it!
Retreat spamming is a major issue right now. You can use it in any situation to win any battle at almost any odds. I conquered a castle SOLO (well, my wife was with me) using this tactic. My equipment was a bow, 2 quivers, and a single 2h axe. Through retreats and repeats, I restocked my arrows and took down the entire defending army until they were few enough for my wife and I to storm the castle and clean up. In Warband, retreating had penalties, and you could not retreat from siege defense. Retreating from attacking still lost you a handful of units in the process. Although... I can't remember if that was in Native or a mod...
In any case, retreat needs sacrifice, and you should not be able to retreat when defending in a siege.
I agree that retreating needs sacrifice, but it should be handled the way warband did in that casualties were based on proximity to enemy. i.e. if you are no where near the enemy you could retreat without penalty on the other hand if they were close you took damage leaving. I think the ability to retreat so that you can pick the terrain (especially when the attacker and or the faster party) as you can start in some unhelpful positions like the middle of a river or in a dense wood in an area marked as plains otherwise. Also now that they have removed the in game banner that let you access and resupply your inventory it's the only way of replenishing you arrows etc. I really missed the ability to stock up on arrows mid battle.