What can I say, massively powerful. Assuming you start near other vict... I mean other civilizations.
Mongol Horde: This never seems to come up enough for me. Maybe at Deity level, but at King level just not so much. But my game is still early, I will most likely encounter some knights later. I'll just add right now Mongols should probably be played one difficulty level higher than you normally play. My game is turning out to be so easy that I don't have a lot of desire to finish it.
Ortoo: Very powerful, what's not to like? Only downside is it can take a while for your first trader to finish his trade routes. More useful when you have multiple traders, but if you are concentrating on war, it takes a while to get multiple traders.Yes I know you get the trading post immediately, I'm referring to getting your trading post up with your 2nd and 3rd victims.
Keshig: I believe the range strength is the same as a crossbowman? I don't have the game open to verify, but I believe so. It's just an ultra fast crossbowman. The AI can't seem to handle it though. Speed kills in this game.
Ordu: see above comment about movement. I don't have the game open to compare base production costs. It took me a while to get mine up and running. I ran Veterancy for my last one, but I ended up not building any units out of that one (building a wonder instead).
Horse Lord: This actually works better than other civilization's agendas like Norway's. It makes more sense that they would like you having smaller cavalry forces. Where as Norway hates you having a small Navy.
Okay so this game is on King level, continents standard size. Plains hill start as you can see, though you can move, though I don't know why you would. I know plains hills have history with civ, but it truly does make a bid difference. Not sure I could have fended off my first attack without that extra production. Unfortunately this game has gotten easy real fast. I won't spoil who my neighbors were, but they were cultural, and not very difficult to take down. It also helped that they were at war with each other before I attacked, and had taken out many of each other's troops. Despite that, this isn't an easy start due to proximities. I made friends with someone to help keep the peace until I was ready. I could have done an archer rush, but that's completely out of character, and I won't do it out of principle. I instead turtled until my Keshigs were ready.