The key to Kiev is settling your cities to both encompass many resources that give food, health, and happiness (wheat, barley, honey, etc) while also making prime use of river locations. Build tons of farms and take every civic on the second row (feudal ones). With farms giving +1 hammer, commerce, and food, that's all you need. Also click the emphasize food button in your cities so they work all the farms first and grow up to size ~18.
I've found that spears aren't the best way to counter barb horsemen. Spears aren't as strong vs cavalry in this mod as they are usually, and usually have 50-60% chance of winning on offense (much stronger on defense though, as it should be). With the exception of the Seljuks (you can't match up to them with knights in 1071, so I had to defend with Tagmata + Varangian mercs, Guisarmiers, and Arbalestiers in cities with walls + castle, but I got my land pillaged), you're better off building barracks and stable in major cities and churning out 6xp horsemen with flanking I + II to fight barb horsemen. You either retreat or win, and then can mop up with slow units or more horsemen. This is especially necessary as Kiev, as you can't afford to have your land pillaged and the "flanking I + II" strategy prevents that.
With Bulgaria, Absinthe's strategy works (maybe it's something about founding Varna and Tarnovo, since I do it too and I've never had issue)
Taking Hadrianopolis and Thessalonica are actually much easier than Constantinopolis- aside from the 50% from walls vs the 225% from walls, castle, and Theodosian Walls, Constantinopolis is also on a hill and across a river, so it's really 275% bonus. However, if you mine xp for your Konniks from barbs and get to flanking I + II and Combat I, then you can even take Constantinople without cats if they don't defend it well. Otherwise, you should spam cheap cats, bombard, and then suicide them to at least collateral most of the units down to 25% of normal health. If you get the 1st UHV you pretty much get the second as long as you build some Orthodox buildings and missionaries (actually I never even needed the Theologian points), and then it's all up to stacking your cities with defensive units to fight the Mongols. Even if plague hits at the same time, it only hurts the weakest 3-4 units in a stack and damages the barbs besides, so not an issue unless you don't have many troops.
Absinthe, another bug I remember from some games of 1.1: sometimes, the yellow message prompts you that mercs are ready for hire and then they aren't there when you open the merc screen (and no its not because I don't have enough culture). Also, I noticed I could hire Almogavars in Thessalonica. Is this intended?
Cheers guys
