Below are the different strategies against the Marshal.
Longarm's attacks are not very dangerous, as he brings a moderate force to test you, unless he is doing particularly well. Archers can be easily dealt with with any troops. Swordsmen are not hard either, as they move very slowly and a couple of fire ballistae can handle them. Trebuchets may be troublesome, as they are set well out of range, so sally forth from your castle and destroy them with hardy fast units such as macemen, horse archers or knights.
Between these attacks, knights will attempt to destroy your farms, quarries, iron mines, and any other buildings outside your castle. To defend against this, use several groups of macemen backed up by archers to guard these areas. Macemen can react quickly to knights and can slow them down and deal out damage to them while the archers can fire upon them.
Longarm's castles are easy to cripple and siege. However, do not try and simply rush in with troops through the open route he has given you to the courtyard, as they will have to walk around his archer filled towers while being fired upon, quickly becoming injured. His knights, swordsmen and archers will also create a deathtrap around the keep. Instead destroy his archer-filled towers from a distance using a few catapults or trebuchets then remove every archer from the keep using about 20 fire ballistae to soften up resistance from a distance. Then use the fire ballistae to try and burn the interior of his castle, frying knights and peasants. Wait until the fire dies down and move in with 20 assassins for the kill or use the open route at the back of the castle, sending macemen.