I defended this castle on the Very Hard difficulty and moved all of the soldiers to the inner part of the castle, on the small gates and the keep. After killing the macemen, the archers and crossbowmen mowed down any enemy spearmen and archers crossing the diagonal wall.
Both gates sustained minimal damage, and they were only taken from the macemen.
In the end, I lost 6% of my total troops. Two archers died because the remaining enemy archers did not run toward the keep.
As for other aspects of this submission:
The landscape is very bare. No eye-catching details were used and the map looks very unnatural. Try sprinkling smaller details, like rocks and shrubs around, rather than clustering them all up in one spot.
Mapmaking 101 is a good introduction to mapmakers. http://stronghold.heavengames.com/maps/index_html
If you have other design problems or questions, check out the forums.
There is no story and there are only basic instructions (rather suggestions). Try adding a backstory as to why this castle is being sieged or a history of the castle itself.