When the siege begins the defender has the advantage. You will have over 3500 points AOC Gold and it is the job of the attacker to overtake your score. The only way to do this is for them to burn down your structures. All you have to do is stop them or delay them long enough for the two hour attack window to end. Your main goal should always be to take out any siege weapons as quickly as possible.
Even if you suffer heavy loses while doing so. Anyone attacking your keep will need to bring down structures in order to win, so if you can stop them from doing so it will not matter how many of your guild they kill Age of Conan Gold. Stop their siege engines and you stop them. Anytime your enemy brings down one of your structures they will gain a fair amount of points and you will lose some. So the balance can shift quickly if you do not stop their siege attacks.
Always end your battle on a good note: The most important thing for the defender is to hamper or hinder the attacking forces efforts to bring down their structures at all cost. Without the points earned from destroying your structures no siege can succeed.