Miklos - good point, and as I mentioned we will improve things a little bit - fixing the issue where arrows just don`t hit archers on walls, instead we`ll keep the defense bonus. However the archer on wall should have a 6vs1 advantage. You will be able to minimize this advantage by hiding your attacking archers behind siege tower, and in the near future the siege shield. That should bring it to something like 2.5vs1.
Archers will become much more effecting in the field combat though, but as far as sieges go, in historical sieges an archer or crossbowman on the wall could take out a dozen of units without any danger to himself.
And do give ladders and towers a try. One infantry unit on the walls can clear out 2-3 archer battalions, and once we make arrows useless at short range in March, one melee unit could clear out entire wall of archers.