1.Find an easy mark.The best is someone so silly as to leave his keep outside.
2.If you are high level and have several heroes and have tons of gold,make use of it to unlock Saladin and Edward the Black Prince.They are the destruction duo.Saladin breaks the wall with his ability and Edward uses his ability to destroy the castle.Either than that method,Saladin and Edward are useless in other tactics.
3.Joan Of Arc is a nice hero to use for cheating.You can turn enemy's troops into yours and if they are inside the walls,even better!They stay inside the walls!If you are keen to use her to attack other things,it's okay but I advise against it.
4.If the enemy's troops are outnumbering yours,try to destroy as much as possible before you run out of troops.
5.If you want revenge,always scout the enemy's castle firs.If you click on the guard house,you can see if it is set to attack immediately or hold your ground.If there is a red circle,that means it is hold your ground,if not,it is attack immediately.
6.Make use of troops such as the Longbowman and Chregosiphen,as they are useful.Chregosipens protect troops and siege machines well as they do huge damage.The Longbowman is a sort of super long range archer and can shoot over walls,so make use of it to attack things that are unguarded unless you use foot soldiers.