Just increase the spread of arrows slightly. At closer range, they should be able to aim for weak spots, e.g. feet, gaps in shield walls. At range just have the arrows deviate so you don't get archers jiggle peeking gaps to hit you running up to the castle at 100m. Also add slight accuracy debuff for AD movement, because good luck firing that longbow without decent footing (especially after jumping...).
A big help in sieges would be that shields on character backs blocking arrows. Not blocking melee swings as that would be a break game-breaking. Yes, there will be the people with a shield on their back and one in their hands which can be annoying. They could balance that nerfing ones movement with multiple shields equipped, or just limit so that only 1 can be carried at any time so that if you choose to have it on your back (whilst climbing a ladder for example) you might prefer to have a two-hander or polearm. This could actually promote swords, axes and clubs to function more as sidearms if you are truly after more realism whilst still having a role in the gameplay.