I know this is a super old thread, but figured I would keep the conversation going.
1. As cammcken mentioned, the Guerilla promotions are really strong. In an enemy city has hills beside it, it basically means your assaulting army cannot be counter attacked. My assault armies have one pikeman, and one longbowmen with maxed out Guerilla promotion. This makes then basically invincible on hills. Obviously with no hills around not as effective, but I found hills to be common enough to justify going this approach. Wrost case you can always use those units as city defence in a city built on hills.
2. Forts are super OP in this game, the immunity to first strikes make then a beast when attacking from those tiles. Basically every city I own has a fort beside it near where enemies are most likely to invade. Combine that with a stack of workers that can insta build a fort, you basically can have a very strong defence against even the biggest stacks of doom. Basically enemy stack comes in, by the time they kill your city defence you have a fort filled with a couple of city garrison archers, and the rest calvary, it can kill any doom stack in a turn or two.
3. Related to above, the immunity to first strikes is very very powerful. All cavalry should try to get to level 3 of flanking asap, as it gives this in the third promotion. You get a stack of 7-9 units, with 6 or so as cavalry and the rest as defence (pikemen/longbowmen) and have them all with level 3 flanking, basically means you can attack any city with a 90% chance for success on the cav (aka retreat/kill the enemy) combine this with the collateral of the cav and a stack of shock units, and you can take any city even if they have tier 3 city defence. Combine this with Guerilla promotion on your defensive units in this stack and a hill next to the city and it is super hard to counter. Even with massive doom stacks.
4. If you can somehow build a fort next to an enemy city then you can very easily take the city, the immunity to first strikes combined with shock troops is super OP (In fact i should look to see if there is a promotion they can get that would allow this). On the world map, as english this is how I conquered france with their huge city defence bonuses. Influance driven wars helps with this alot as well.