So, in this mission you face 4 adversaries and you have 1 ally. Marshall, Sir Longarm, Emir Omar, The Abbot of Stirling and Wazir, "The Terrible", The Sheriff of Nottingham is your ally. Both you and all the AI start with 10.000 gold pieces.
This is not an extremely hard mission, but if you still have trouble or just want to see how I build my castle, above is a save that will grant you victory. Anyway, here is what worked for me:
When you start, slow the game down to 20 and build a granary, apple orchard (we'll deal in apples), an armory, 3 stockpile elements so they form a nice square :) and 3 woodcutters north of your keep. The Wazir will fire at them, that's why we need to deal with him.
Don't bother about the Abbot, the Sheriff will deal with him.
Buy 25 maces and kill the wazir, in my game he only killed 8 of them, so I had 17 left, who proved very useful in the next step.
In this mission I chose to make a round moat around my castle, as none of the enemies have moat digging units. So lay down a moat and have ALL your units dig it.
Now place 3 fletchers and let them produce crossbows, due to the little farmland we'll buy all leather armor necessary to make crossbowmen.
To your north you have stone and iron deposits, fill them completely with quarries and mines to finance everything.
The Emir has probably started bothering you with mangonels, return the favor and place 2 round towers south of your keep, front against the moat. Left one with ballista(to counter the Marshalls catapults), right one with mangonel. Place all your crossbowmen on the tower.
If everything is ok, the Emirs initial swordsman attack should now have reached your moat and the moat should just be about finished.
Now stone, iron and wood should come in. You can create an invasion army in whatevery style you like. I chose pikemen. So place all workshops you need, I placed 4 poleturners and 4 armourers.
Now just expand your army and crush your foes, keep producing crossbowmen and an invasion force. There should be little to no resistance from your enemies.