A few things I want to mention:
1. I was playing at Immortal, and my opponent (Russia) had a much greater army then me (riflemen and cossacks). After he conquered one of my neighbour, he immediately declared war on me, and caught me off guard (that was nice!). As he was superior, I decided to leave 3-4 cities totally undefended, and brought my whole armies to a relatively central and well fortified city to wait for him.
The problem was, that his great army had very few units which were at full power, almost all of the units would require healing. But as he saw my cities are undefended, he marched the nearest one and conquered it. Then he left half of his army there and started to attack another undefended city. Meanwhile, I assembled a fair army and went against his army and marched towards him in order to give him some attrition. But he took the second city and went immediately for the third one. But as he divided his "tired" army to smaller and smaller pieces, I took advantage of it and attacked and took back the 2nd city he invaded. Then the Russian army turned back to take the 2nd city back. Meanwhile I got 1-2 turns rest, then left the 2nd city completely undefended and marched to the 1st city and I took it (I had several city raiders, who gives me the edge) Then the russian army took the undefended city and again, refusing to have a rest, attacked directly another undefended city of mine, by using the half of its army. In the end, by wasting very few units, I annihilated the Russian army, as they divided their army too smaller and smaller pieces and they refused to have a rest.
2. Either AI plays too inefficient or the Great Lighthouse is overpowered. Almost the half of my commerce income comes from trading routes. Combined with barter, even a stupid one tile island city can be very powerful (thanks to all the shield income) I like the fact, that trade means something more than a bonus, but commerce from tiles is simply too insufficient.
=> I think barter is too overpowered, and it is the best economy civic until you want to build a corporation. I think, the shield bonus should be dropped to 25% or so while commerce penalty -50% is plausible. Also, building cottages in the middle game should be a more attractive choice. I played till industrial age, dominated clearly, and never built a single cottage. (Watermills and Farms did a decent job, i didn't even built a single plantation till late renaissance, because there weren't any such resources around me)
3. A revolution never happened to me. But the AI can't cope with revolutions. AI should either get smarter or it should at least get a stability bonus for higher difficulty levels. (Or I should have stability penalty for playing in higher difficulties, whatever)