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.