if im not wrong at lvl 14 or so, and over 160 archery lvl. The point is easy;
find a spot where your troops arent hitted by the archer defender, pick a blind point and shoot from there, I use cannonfodder for refill the arrows, swadian militia, I place them a bit far from the casttle and when they are over enemy fire ill pick the arrows, and back to the blind spot.
The enemy shot me in time to time, so i cant (you can but not allways happend) take the casttle on one round, this is why i retreat, and siege again. If you have swadian knight or infantery , when remains only the same numbers enemys than your troops , without archers that is you take down the first, send your troops and hope the best
At open field i allways use the same tactic, heavy charger (atm heavy hunter because my riding skill) and tanke down the rangers with bow, then send muy troops when the enemys is facing me, this tactic works with the most of the enemys . If you are heavy outnumbered you must be very good with tthe aim
even if you dont take the casttle, htis tactic is the best way (without killing looters) for train archery

(imo)