So I've just started my own short campaign, first time DM'ing. It's based around the Sub Romano Brytonnic kingdoms as they dealt with the Anglo Saxon threat. So King Arthur basically but more historical than mythological. His name is not mentioned so they don't realise till the end.
The entire city is made up of only Paladins (with a few permitted multi classes - no rogues). All players are Paladins with multiclasses. They group into Knights and their squires and each knight is the Lord of a separate kingdom with their own revealable back story, some of which they know some of which is revealed by interaction with NPCs.
The campaign starts with a grand jousting match (which I built mechanics for), whose end is interrupted by an invasion of barbarians (Anglo saxons). The players must beat off the invasion, then travel through a hazardous terrain filled with monsters beyond the city to track down the Kings kidnapped daughter, resulting in a massive battle at the end with many high level players.
The whole campaign has a secret honour system based on your actions which dictates how big your reward is as to how much you hold to Paladin morals. There are also secret abandoned places where you can gain a personal level up based on how you get in there. Ie if you break down the door you gain a level of barbarian, if you lockpick you gain a level of Rogue. That kind of thing.
The city etc and some of the mechanics are my own creation based on the history. I even have a Merlin character who skulks in the shadows making sure players don't do anything too mischievous.