This is the result of a number of experiments with the game editor, playing with the landscape and castle building tools. Once i had created a few castles that had reasonably standard layouts, i decided to "think different" and looked further into the future of castle and fort building. With the advent of cannon, the walls of castles were angled to deflect incoming shot, they were also thickened. Killing fields were created so that incoming troops stood no chance of breaching the walls.
The resulting castle/fort has all these features. I also added a few features that have no historical basis, such as interconnected towers and towers that are stand alone and provide some covering fire.
A lot of time was spent creating the landscape, it plays a major part in the way the game proceeds and the way you plan your attacks.
I suggest first playing the siege in the Attacking mode, careful planning and good reconnaissance will allow you to gain access to the Keep with minimum losses. However i think it is well balanced, 'hard' is practically impossible, with some planning, 'easy' is easy.
I have only a few remark. Landscape you worked so long is not, imo, completly realistic. But it's not the worst matter about it. Fact is landscape provide so many blind spots that you can win only with your missile troops and some engineers. About the lasts, it's, imo, not so wise to provide players enough gold to build 50 trebuchets. I played the map only once, at normal, playing at 90 speed, and using cavalery for fun (that was useless I admit). Result was a 4% loose in a long game, mostly build on attrition war. I have no reason to think it would be harder at very hard setting, just more long because you start with less troops. I noticed too, making my cavalery wandering that most ennemy troops didn't reacted. Think to put them on offensive or defensive behaviour, at least for external infantery and cavalery (but not keep guardians). Last even playing less cowardly, understand without using blind spot to cover your own missile troops, I suggest you study impact of siege tower use. However last remark would be more for your future maps than for this one.
Nota : try to avoid to start your scenario in january 1066, that is a bit banal ;)
Remember in middle age,especially for low land fortress, area around castles were regulary cleared from trees and schrubs, in order to make a glacis to avoid ennemies find cover. These men weren't so fouls. I dont say everybody have to make the same thing at stronghold. I just think, if you make exactly the inverse you take great risk. Your current map is the perfect illustration.
Attration war is also often a plague in stronghold siege map to play as attacker. I use a time limit to solve this problem. Point that could stress players or even make them to not play the map at all when they detect this feature however.