I usually don't play earth maps that much as I enjoy the unknown from other maptypes more. But I've done one earthmap game in CiV with the Mongols. Not much difference in strategy there from other maps, the difference is you know where your enemy approximately is. This game I did a domination game, where I quickly run over europe, asia and africa with my Keshiks, puppeting everything but 6 core cities, when ready with overseas capabilities invaded america and did the same there.
I did also abuse the Mongol city state bonus here so the first thing I did was to puppet the closest 3 city states which meant I didn't have any city state friends for the rest of the game, but well, this game was on King level so you survive quite well without the city states, I suppose the same would've been possible on Emperor. Mongols are a bit to strong with the Keshiks against the AI who simply cannot defend against quick ranged units. So for a fun, quite easy, conquest type game, go with the mongols, do some early conquest with Horsemen, then when Keshiks are available, do your major offensive, with two armies with about 5-6 keshiks in each with a horseman to capture the weakened cities, accompany each army with a Khan and you can quickly run over the continent. Puppet all you conquer and happiness will not be that big an issue. Though, when the patch arrives this might not be as an effective strat anymore.
