Exactly what Biervampir [AUT] said, although I will also add some other details:
Even though they do not need sleep, they will still make lairs. I think they sleep only to recover lost health but I'm not sure about that, and I think that if the Empire has access to Workers (due to giving them a Dungeon Core or some Spirit Workers) they will try to rescue them back to their lairs.
They don't have Paydays.
They do obey the same rules of population and room attraction. You can create a fortress with no units, but many rooms and some gateways, and it will be filled in a matter of minutes.
Speaking of attraction, if you give the Empire a Beast Den it will still attract beasts normally. Of course, since the Empire doesn't research and spend Sins (unless you give it a Dungeon Core), you will only see Skargs and Oculus as foes.
Rooms function normally and minions do work in them. Juggernauts will charge Garrisons, Warden will produce parts at the Forge (even though they would be useless), training would still happen at the Barracks, and so on... Crypts, Torture Chambers, and Sanctuaries can tecnically work but you will rarely see their functions in action for the obvious reason that the Empire doesn't have Workers to collect corpses and prisioners and a hand to drop them at the Torture Chamber. (Unless you give them a Dungeon Core with a AI)