Just be forewarned that the last patch introduced a lot of new bugs. Some are more amusing than problematic (enemy Civs berating you in 1500 BC for not having developed nuclear weaponry yet), but some are pretty bad (the AI offering deals it will refuse to accept, the AI horribly undervaluing great works). Two steps forward, one back…
Maybe I should start a game as the Vikings and see what the music does. 
No, they’re mocking you for not having “new, clear” weapons, obviously made from nicely polished steel. Are you really going to war with those dingy old iron or bronze numbers? For shame!
The Scythian ability to self-heal when they kill somebody is pretty nice, but their early special unit (getting two worthless horse archers for the price of one) has always seemed very weak to me.
However, there seem to have been a couple patches since the last time I played as the cranky horse lady, so it may have been improved a bit. I notice that they’re back to calling me a war-monger after declaring war on me…
Funny, I haven’t had any trouble with the latest patches. I’ll have to see if I can provoke some bugs…
I just finished my first game (played random, got Alexander on a large map) - took a while to get the mechanics down compared to previous versions and then realize to play as Alexander the best bet is just to accept everyone will hate me as a warmonger and go to war constantly.
I did run into both of the bugs mentioned above - Brazil taunted me in the ancient era “Can’t you see nuclear weapons are the future?” and I had a diplomatic penalty about it with them the rest of the game (based on their “Nuke Happy” hidden agenda, I guess). Also, countless times the AI would offer a deal but say they can’t accept it. This would be really bad when they were suing for peace, but say they can’t accept the deal the offer - when I suggest they make it more equitable their counter offer includes me paying hundreds of gold to them per turn.
Overall, I like it. Oddly it runs much better than Civ V ever did for me - the graphical changes play nicer with my system, I guess.
Horse archers aren’t great (since they lack the 2 range that regular archers have) but they aren’t useless either, especially since you get two of them for the price of one. And getting two for the price of one is a civilization ability that applies to all light cavalry, not just the horse archers - so it’s very useful later on when you unlock Cavalry, for example.
how does espionage work this time ? and is the American civ worth anything ?
I dusted this off this week since I have the week off, and played through a whole game - earned a science victory (in the past few games, I kept “winning” a cultural victory just before whatever other victory condition I was heading for was met). Earned the oddly specific “Man on the Moon” achievement - Win a science victory, while occupying an Egyptian city and having activated both Newton and Einstein.
I did notice that the major bugs I had seen before appeared to be fixed. The AI diplomacy was better - it wasn’t offering trades it wouldn’t accept, it seemed to value great works properly, and it would actually accept alliances. I didn’t check to see if you could still get the AI to give up everything in exchange for a Joint War, but I suspect that’s been fixed, too.
Having alliances was interesting - until one of my allies decided to start invading the city-states i was suzerain of, and I couldn’t declare war on my ally to stop it.
Assuming you still care about the answer to the first question, It’s a long explanation. Short answer, make spy units, position them in cities, and have a choice of spy activities based on the districts in the city.