Here I come.
The Mongol Units are great, so... yes, the Chosogabe are soon going to be a distant memory -I only played five though, so you'll have the pleasure to complete the massacre. In fact...

I really stole a few hours to play these turns right before leaving for the train station, but I enjoyed it soooo much...
Pre-turn: 500 AD
Check our neighbours. Almost everyone knows Invention, so why not us too. Buy it from Khwarizmia (the weakest civ to know it) for 1120 gold.
Since I am here with this new knowledge, I cannot help but upgrade our Mangudai Bowmen to Khorchin.
Ok, then, not much else to do other than declare war on Chosogabe and begin the bloodshed... I did give them a chance -but they refused to surrender all their cash for a renewal of the peace deal. It's all their fault then
Turn 1 - 510 AD
Our sailing ship contacts the Arab tribe. They are already furious with us.
The first Mongol riders land in Chosgogabe territory. Now let's see what the Choso will do...
Hurry a couple of temples in former Baekje lands.
IT
...nothing. I see three Chosogabe horsemen galloping next to us, heading for some other place. Maybe it's time to build a few embassies.
I have left two galleys in Arab and Tokugawa lands. Neither of them is happy about that. Yes, they will go immediately.
Pyongyang riots and gets a taxman.
Turn 2 - 520 AD
Move some of our wonderful UUs towards the boarding point.
Then it's party time: attack on Uwajima.
I start with some bombardment -there are two regular defenders, both of which loose one hp. The Temple is destroyed and trhee or four bombardments fail.
Then the attack:
Veteran Bagatur vs 2hp Footman, is redlined and retreats (bombarding with his spare movement points)
Veteran Bagatur vs 2hp Footman wins flawlessy, attacks again -is redlined but wins and promotes to Elite
Veteran Bagatur vs Samurai Archer: two bombard rounds reduce it to 2hp, then the attack is won.
Uwajima is captured with one worker, 4 golds and 5 resistors. Our UUs are so nice
Attack on Tokushima.
First round of bombarding uses one movement from each unit. Three defenders, all down to 2 hp. City size drops from 7 to 6.
Attack: Veteran Bagatur vs 2hp Footman wins loosing 1 hp
Regular Bagatur vs 2hp Footman wins flawlessly; attacks again, is redlined but wins and promotes conquering the city.
Veteran Bagatur attacks wandering regular horsemen, in the open, and dies miserably
Veteran Bagatur attacks extremely lucky but redlined horseman and kills it.
New Erdenet and New Tsetserleg founded in suggested spots. Barracks.
Establish embassies with Rajaputana, Ottomans and Russia. It looks like they are all at peace, and this is not good.
Ottomans' capital is on wealth...
IT
The French complete Sistine.
Turn 3 - 530 AD
Set units to attack Tosashimitsu.
Establish embassy with Vikings. They are 5 turns from completing Sun-Tzu.
Move troops out of Tokushima and a Uwajima, out of fear of a flip.
IT
Tokushima doesn't flip back -it is retaken by Chosogabe; of course, if I leave it undefended...

I didn't think they could make it right into the city, though.
Everybody is building Leonardo's workshop.
A Russian and a Chosogabe ship approach our Baekje district.
Turn 4 - 540 AD
Attack on Tokushima
Vet Bagatur kills Horseman and Archer and promotes.
Vet Bagatur kills redlined Footman (from the bombardment, of course

) and takes the city back.
Attack on Tosashimizu:
Bombarding more or less ok, defenders all have 2 hp left.
Vet Bagatur vs 2hp Footman wins and is promoted; attacks again, looses 2hp but wins and captures the city.
Kill four Choso horsemen and a Samurai Archer in the open
IT
I stupidly left our Bagatur in the open field, and Chosogabe horsemen kill two units, loosing one of theirs (our defender is promoted).
Chosogabe land two horsemen near Seoul -right between two of our Swordsmen, two Ordu Archers and one Korchin...
Turn 5 - 550 AD
Venture with a few Bagatur and Turghaut into Chosogabe land, and see Niihama. Four Bagatur *may* be enough to take it.
More troops land in Tokushima.
Found New Baruun-Urt
Notes
Chosogabe would take peace now, but would not give cities for it.
Several troops have just landed, ready to take part in the assault (let's hope Tokushima doesn't flip again!). More are coming from our mainland, where very few have been left in defense (sorry, I might have left a few of them on auto-move). Chosogabe seemed to prefer to attack our Korean territory.
Our Galleys are presently more than our rate of production.
A settler is in position to found the last city in our 8-distance ring.
I was also itching to start a war between the other civs, but I decided to wait until we know the world map a little better.
Well, that's it. My mind is already on the airplane.
It was good playing with the Mongols. I'll be eager to follow your progress and come back and read all the story in a while. Have fun mates!
--- Here is the save ---
The Chosogabe lands
And a world map