If you have a completed save file from Kingdoms Under Fire The Crusaders you can play as the original heroes in custom missions.

After the credits roll when you beat a campaign, it will give you the option of making a new save. If you do, it will allow you to replay any stage in that campaign with your previous stats. Enemies aren't more powerful, but you can continue leveling up your characters after each mission.

Beat either Walter's or Leinhart's campaign.

Beat either Rupert's or Cirith's campaign.

Beat Ellen's campaign

Beat Morene's campaign.