Situation[]
Sir Edwin attacks the castle of Sir Grey, or at least he tries...
Instructions[]
Sir Edwin is getting a nasty surprize
If properly prepared, this mission is laughingly easy. The only preparation neccessary was to position your troops on Sir Grey's estate beforehand. If you didn't do so, you'll have to fend off a full siege, because Edwins troops start marching immediately and will be there before you.
Put your melee troops (the spearmen and Sir William's 5 swordsmen) near the forrest on Sir William's neighboring estate. That's were Sir Edwin sets up his siege camp. Just hack it down before he starts recruiting catapults.
The rest of his invasion troops are no match for your archers.
Alternative approach[]
If you've followed this path, you should have more than 400 archers, preferably positioned, like shown on the picture. If so, this mission will be over after about 45 seconds and your Archers will mow down Edwins army so quickly, they never manage to cross the bridge and the siege camp will never be built.