The king wants to strengthen the Royal Guard and for that he needs swords. He puts you in a production race against Sir Edwin Blackfly. Whoever accumulates 100 swords first, gets the contract.
You are given back your estate Harbury and you'll notice that the AI has done some redecorating. Some bread production has been set up and a armourers shop has been placed. Delete the armourer as you can't recruit swordsmen or pikemen anyway.
To make swords you need iron. You have an iron deposit in Whittle and one in Wintertop (which you freed from the wolves a few missions ago). Wintertop has been given to you (probably as a payment for your endlessly doing Sir William's work). Since Wintertop is right next door, it is easier to get the iron from there, but if you wish you can set up a second production at Whittle to speed up the process.
Build two additional iron mines and four ox tethers at Wintertop. Squeeze in a second and third apple orchard near their granary too. Task their carters post to deliver the iron in batches of 20 units.
Back in your home estate build eight sword makers. You may wish to delete your pole turners huts to free up building space and work force.After a game year you'll be told that Edwin cheats and bought 25 swords. He'll charge ahead, but don't panic. His sword production will stall completely about halfway into the mission, because his economy breaks down, due to peasant unhappiness.
Meanwhile continue to make Archers and bring them to Sir Grey's castle. Let them man the towers and walls. Also bring your surviving veterans from the former outlaw camp to Sir Grey's castle.
Now sit back and wait for the swords to come in.
There is nothing much alternative to do in this mission. Just keep pumping out archers and send them to Sir Grey's estate. If you've been following this approach, you should have something like 400 to 500 archers on Sir Grey's walls and on the ground.
Squeeze in a few more pig farms at Slaughterford to prepare for mission 12.