With my own Nord infantry, the best tactics I have found against a mounted enemy, or an enemy that has some mounted and some foot troops, is to have them hold a position, and stand close together. If I can get them onto favorable terrain, like on a hill or a river bank, then I will do that, although that is not always possible. I have about 30 Nord infantry, mostly Huscarls, Veterans, and Warriors.
I then have my mounted troops follow me while I repeatedly rush through the enemy formations to break them up. I do not want a wall of horsemen hitting my infantry, I want them to trickle in a few at a time. Some of the enemy mounted troops will hit my infantry, and some will play chase with me and my horsemen. The ones who are playing chase will suffer some losses to my horsemen, and the rest I will 'drag' a couple at a time into my infantry.
The clustered Nord infantry do not have any trouble handling horsemen, even Swadian knights and men-at-arms, if they can absorb their charge, stop them, and then swarm them. Sure, it might take a little while for them to bring one down, but bring him down they will. That is why I have them stand close together. If they are in a line, it is easier for a galloping horseman to break through them. If they are grouped together, and especially if I drag the enemy horsemen so that they hit my line end-on or at a steep angle, there are enough infantry in front of the horse to stop the charge.
Once the enemy horsemen are gone, I have my infantry move up and engage the enemy infantry. I and the horsemen will trample and kill as many as we can while the infantry close with them. I have yet to run into any footmen who, especially when disorganized and harassed by my horsemen, can stand up against experienced Nord infantry.
For sieges, I had to learn to hold back and keep myself out of the battle. The temptation was to rush in and try and win it myself. I lead the first group down beneath the walls, and have them spread out, and then stand there and duel with the enemy archers. That is another nice thing about Nord infantry -- many of them have bows and so double as ranged troops. With only a few dedicated missile troops mixed in, we still send up a heavy shower of arrows and bolts. Once we have cleared a few men off the walls, and I see the hail of arrows from my side dwindling (as my infantry are running out of arrows), I order the charge.
My job during all of this is to keep my shield up and take as few points of damage from enemy arrows and bolts as possible. If I can pick off a couple of the key enemy archers in the flanking towers myself, I will do so. Otherwise, I will stand there and let successive waves of troops go up the ramp and kill the enemy. It may take several rounds of reinforcements before they clear out the bulk of the enemy troops. I will then go up myself, and have the men follow me if there are some defenders that they are not going to get to on their own.
The reason I save myself is for the final fight in the keep. The machine likes to send me in there against odds, and sometimes with nothing but a couple of frigging skirmishers to help me. If I go in there with just a few hit points left, and lose that fight, the whole attack falls apart and we retreat and start again outside the walls. I am just not a good enough melee fighter myself to win against odds when I am already seriously wounded. Like Dirty Harry said, a man's got to know his limitations.
Heavy losses during a siege almost seem inevitable to me. Perhaps there are other players who have learned tricks that I don't know to minimize this. So, my strategy for a siege is to have a large siege force that is mostly made up of the troops of other lords. That spreads out the losses, and leaves my army with only a few casualties. Having Jeremus, and having boosted his medical abilities as much as possible, is a big help also. If anybody else knows a better way to win sieges, I would be happy to hear and learn their tactics.