The siege of the Castilian fortress of Toledo, in Spain. It is an impressive castle, perched upon a hill, with a river running through large portions of its circumference.
The Portugese have brought a massive army, with many impressive siege engines, but because of the direction they chose to attack from, the only gate they can attack this side of the castle is this pitiful one cramped in the corner of my walls.
Then they have to travel up this long and lonely path to capture my city center, all the while my troops will be harassing them from all sides.
Unfortunately, while terrain and circumstance favor me against the vastly more numerous enemy, my troops have loaded in the very opposite side of the castle! It is a race for whether the enemy can breach the walls before my troops get in position. Don't forget to notice the gorgeous river running aside my castle.
Well anyway, I won that siege, but barely. The castle/terrain engine in this game is amazing. I can't wait to fight with larger castles.