It was my first scenario ever and I took the challenge on Deity. Ended up with Denmark (spawning far east) and I found it to be rather easy and I'm definitely not a Deity all-star. Finished in 53 turns. On the other hand, I snagged London as the english AI had already built it's 6 UB and it was probably 2-3 turns ahead of a loss to england making me think it's quite possible as england as well

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Seeing how my game turned out positionning-wise, I would suggest you to consider defending one primary flank in which you will build your 6 buildings and then rush building the doomsday book in London. The doomsday book can be built in 3 turns if you buy forest tiles and spam cut wood with 4-5 workers so the biggest challenge is building the 6 doomsday w/e building in other cities.
The NW flank of London seems to have the best production lands (and some valueable iron for defense units) and the most cities that are at 8+ tiles from london so I would likely concentrate my building production there, using every other city to make crossbows or LS/catapults as iron allows. Make sure watch your iron use so that you don't end up with the resource penalty at a bad moment.
Focusing building production there means having your most defense there, even if it means losing a couple cities to the south and east. On offense, I didn't discover any CS so I used my gold to either purchase workshop (to increase the production of doomsday building) and once to rush a berserker on the island rather than having to wait for it to travel 7 turns.
I might try and respawn it over and over 'till I get to play as Harold. As far as offense goes, I would think that the one to the east (denmark) and the one to the south are probably the best 2 contenders to a Deity win because the progression from the north has so many cities on the way to london that chances are you will have lost to a Deity Harold well before you make it to snag london. In my game, the guy to the north had the biggest army and lands throughout all of the game yet we first met when I stopped my progression towards the NW (had picked my 6th city atr 8+ tiles) and I have already had london for a solid 25 turns at the time.
I'm guessing the defense is probably much more stressful though :s, still Deity felt pretty easy as Denmark. Much easier than any Deity game I've played since patch.
Got 3 achievements from that one alone -_-
*edit* after 15 resets I still didn't spawn on defense yet so I just give up and wish you the best of luck~. On the other hand though, I will retract about the offensive positions. The one that spawns further north has troops starting off on the continent right away so it can definitely compensate for the fact that london is sightly further. Plus he can spend 500g to CSs and get like 8 free units instantly. That's pretty solid. So it is definitely Deity doable on any of the offensive civs.
*edit* ok I feel stupid now I found how you can actually select your civ so I might try Deity as defending harold later tonight.
Ok the Deity defense is definitely way harder. You begin w 4 units and all 3 AIs are starting right beside your lands rather than having to travel for ~6 turns like I had to when playing on offense. I settled a GG into a citadel and it really cleared a pack by itself, it was definitely more than I could've gotten out of a GG with these few units spread. But I'm definitely not to the point of doing this perfectly. This is very damn rough