I guess you've never heard of Theocracy, Vassalage, or Charismatic leaders then. The combination of the Keshik for extremely early conquests and the Ger all the way to Cavalry makes the Mongols excellent warmongers.
Barracks + Stable + Vassalage + Theocracy = 9 xp, still short of the third promotion unless you are Charismatic. Ger makes that 11, an extra promotion. If your leader is Charismatic, 8 is the third promotion and 13 is the fourth, so the first Great General you get will have you getting four promotions on each brand-new mounted unit at your main troop factory city, and until then you can switch out one of the civics for a more productive one and still have 3 promotions at all your production cities.
With the addition of espionage, I'd rate the Citadel as the worst UB; Jails are no longer gravy, but Castles still suck. Castles just come at the wrong time, and I pretty much never build them. Replacing defensive units with better ones, or building economic buildings, or building something for culture for defense that won't go obsolete, are a better use of hammers than a building that will go obsolete so fast. My games break down into three categories:
* I plan and start an early war. By the time I can build Castles it's over, and I have switched to growth mode to get to gunpowder first.
* I get stuck with an early war I don't want in a game where I was planning to expand first. I've already lost, Castles might delay it, but while I'm holding off the horde in that city with the castle the uninvolved civs are springing ahead in tech and economy.
* I don't get stuck with an early war, and achieve economic dominance on plan. Castles will be obsolete before I start conquering.