OK, enough talk, time for action!
Re Charis' last post - the sword in Caesarea would take 4 turns to reach the cost and load onto a Dromon, so it can't get to the target any faster than by walking.
Peeking in the editor, the Bulgars' other king is in Pitesti, all the way on their far side, which isn't unexpected. After Pliska is taken, another city besides size-1 Pitesti will likely become the capital, and the king may move to it. A fast TKO isn't going to happen, so I'll go with the ultra-blitz plan. We'll attack most likely on turn 9.
Research set to Norse Tradition at minimum.
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843 AD: Start by moving one Dromon east to pick up the sword from Sinope. The other goes westward to seek contacts.
Move the lovely Theodora king unit over to make contact with the Bulgars. We establish an embassy.
As expected, there's the one sword, one horse, and the free spear. It's building a settler as I guessed. Their king unit has a HP bar and is doing police, which means it's a 1-1 unit, not 0-0. I think we can handle it, though.
Our plan is locked and loaded, with Constantinople set to build an archer at 10 shields/turn. I didn't realize this before, but Adrianople and its 3 shields/turn can also contribute an archer before we're ready to attack.
Not much to do at the other cities. Order up mostly workers and send out units to bust fog. Athens starts a settler to claim iron. One of our 1-1-2 kings will go east to seek new life and new civilizations.
Worker in Athens moves to start building roads to connect Greece and Byzance. Something's screwy in Athens - the city is pulling 5 commerce from terrain but somehow has 8 showing in its city window? If I move laborers around, that extra surplus of 3 remains.

Also, the Greek cities count as connected - listed on F2 as trading cities and they have the home luxuries - although there's no road?
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Between turns, the Bulgars move their horse out of the city northwards.

I realize that it would be a very good idea to park a unit on that mountain to keep an eye on their capital.
846 AD: More of the same. Moving swords and boats, busting fog. Note that the Civilopedia doesn't mention that Dromons sink in sea water, but according to the editor they DO.
849 AD: Yet more of the same. Two Bulgar swords appeared out of the fog, I can't quite figure out what they're doing.
852 AD: A Dromon contacts the Magyars to the north. No trades to be made, naturally. Pliska has dropped to size 1 as expected.
855 AD: Pliska back up to size 2. I haven't seen a settler leave it, though.
858 AD: One of the wandering Bulgar swords left the city along with the settler. I don't know where the other went but it wasn't into Pliska.
We could march into Bulgar territory this turn, with three swords, two archers, and one 1-1-2 king. Or we could wait two turns and have two more archers. I decide to spend the 24g to investigate Pliska again to get detailed info.
OK, so we'll have to kill four units to take the city. That means waiting for those two more attackers.
861 AD: Our boats contact Burgundy and the Turks and an exploring spear contacts the Abbasids.
864 AD: It's time. We declare war on the Bulgars, and move 3 swordsmen, 4 archers (3 veteran), one spear, and one king up next to Pliska.
Between turns, no counterattacks.
867 AD: Great news comes from our front-line parties! We have contacted Spain, France, and Castile!
Oh, you want THAT news.
In the Battle of the Bulge..ars....
Vet archer attacks a veteran spear and wins 4-2! Another veteran spear appears underneath (uh-oh...)
Next vet archer attacks that spear and wins 4-1! Luck be a lady tonight.
Last vet archer attacks a regular spear and loses 4-1. Oh well.
Reg sword attacks reg sword and wins 3-2 (whew!)
Reg sword attacks 2-HP spear and wins 2-1.
Last sword attacks the King and loses 3-2.
Reg archer attacks the King and...
We keep the city. I don't fear a flip: our culture is far ahead of the Bulgars' (we started with two temples and a colosseum). It comes with two workers and a barracks.
870 AD: We add Cordova to our list of contacts, and fortify around Pliska. A Bulgar swordsman is going to attack the city this turn; we have a fortified reg spear defending; if we lose it, oh well. There's also an incoming archer and warrior but I'm sure we can pick those off at will. (Incidentally, that Bulgar swordsman is only there because of the AI free spear. That sword escorted the settler to found Veliki Praslav, and when that city got a free spear, the sword was freed up to come back.)
I'll pass off now. Was only 9 turns, but I had plenty of fun, and that evens up the year numbers and lets next leader veto the initial 10-turn worker builds if he wants.
Map in next post. Take it away, Arathorn!
http://www.civfanatics.net/uploads6/rbc13a-870ad.zip