Turn 83
It looks like Yuris has abandoned the city for now. I easily take the city back:
Notice that I have put the commercial hub and Hansa down again in Frankfurt. I might lose the city soon again, but if not this would lock down the costs. Moreover, the commercial hub here actually provides the adjacency bonus for Mainz even before it's complete, so for this turn at least, Mainz will have 2 extra production from that.
Since Frankfurt has fallen before, the trader going from it to Aachen draws stops working that route and is available for another route, so I transfer it to Ulm to start a route to Aachen next turn.
Turn 84
First thing, Mathematics is boosted since Aachen finishes its Campus. I activate the great scientist there to build a library instantly and my science per turn goes up to 26 from 22.8. Secondly, I start the trade route from Ulm to Aachen and it gives Aachen 2 food and 2 production (due to the completed campus and Hansa at Aachen). Aachen starts working on a builder (3 turns).
The warrior in production in Mainz is out. I start building the ancient walls here (3 turns) to get the boost for Engineering and to have more defense here.
Frankfurt falls again and my warrior is destroyed in the process. I immediately counter-attack:
One archer promotes this turn. Another archer shoots at Yuris' archer (ignore the crossed red arrow - I marked where that archer shot wrong first). The warrior is ready to take Frankfurt back next turn.
Sullla's science is 23 compared to my 26 now, so I might be able to tech a bit faster for the time being. Archduke is at 3 cities currently.
Turn 85
Iron Working is completed, but without iron, I can't produce swordsmen, so the builder out of Aachen will go to the iron near Ulm. Next tech is Engineering.
Cologne produces an archer who moves towards Mainz immediately. Next build is a heavy chariot (4 turns) because I feel like Frankfurt will fall again once I capture it and I'll lose my last non-archer unit so I need another one. Heavy chariot gains 1 movement bonus if on flat terrain, so it can come to the center faster.
In the center, all the archers shoot at Frankfurt and the warrior captures the city back. The garrisoning archer is destroyed in the process. I do the same trick with the commercial hub to boost Mainz's Hansa.
Archduke is down to 2 cities now, so Sullla will probably finish him off in 5-6 turns. In terms of military strength, Sullla is leading with 363, followed by my 133, Yuris' 117 and Archduke's 59.
Turn 86
Well, Yuris take Frankfurt back, and it will be 5-6 turns before I can take it back since I don't have any melee or cavalry units.
I shoot at the city center and the closer eagle warrior with my 3 archers while the 4th one promotes. The new archer arrives in Mainz, and my other archer is down SE of the bananas tile. Yuris promotes both of his eagle warriors with the +defense against ranged attack promotion, so my archers don't hurt them as much as you would expect.
Not much happening in the rest of my empire. Same builds are going on.
Turn 87
Mainz finishes its ancient walls, and the boost immediately finishes Engineering. I start an archer build here.
Sullla's troops have arrived from the west. He attacked my promoted archer last turn and Yuris attacked the archer SE of Frankfurt with his eagle warrior.
I make a small retreat to the east. I move 2 archers on to the hills near Mainz. The badly damaged archer withdraws one tile and shoots at the horseman, and the archer next to Frankfurt shoots at Yuris' archer. The circled archer promotes this turn. If Sullla decides to come closer, I'll have at least 4 archers to shoot at his horseman, so Mainz looks safe now.
Aachen finishes the builder and it start moving towards Ulm. I start building another heavy chariot here.
Archduke is down to a single city now...
Turn 88
My badly damaged archer is destroyed by Sullla's archer, but no other casualties. Yuris effectively abandons the city as he withdraws his eagle warrior and garrisons Frankfurt with an archer:
I attack Frankfurt with my 3 archer to keep its health low for my eventual counter-attack, and harass Sullla's archer with my promoted archer.
Here's the overall view of my empire. Ulm is still working on its infrastructure, but the rest are producing units.
Turn 89
First thing I notice is Archduke is gone! Well, Sullla can focus on me or Archduke now easily, so it's more urgent for me to secure Frankfurt and its vicinity now.
Cologne produces the first heavy chariot. It moves towards Mainz immediately. Mainz produces yet another archer. I decide to start a spearman build in Mainz (4 turns) to counter potential horsemen of Sullla and yet another archer in Cologne (you can never have enough!).
In the center, ny archers keep shooting at the city center since there are not other targets available now (Yuris has withdrawn all his troops).
Turn 90
I get a boost to the Mercenaries civic since I now have 8 land units with the completed second heavy chariot in Aachen. Aachen starts working on its Commercial Hub (4 turns) since I need extra traders and the gold to keep up with Sullla if I were to not forfeit the game now to him.
The builder mines the iron next to Ulm finally. I still can't produce iron-requiring units since I only have 1 iron hooked up, but if I can get encampments, those will allow me to get swordsman.
You can see the 2 new encampments circled in red and the new iron mine in blue. Aachen will have to wait for the next turn to put down an encampment since it needs 7 pop for its 4th district.
The heavy chariot is in position now to take the city next turn since the city health is basically zero after 2-3 turns of constant archer barrage by me.
Turn 91
Frankfurt is back in my control. One archer softens the city (0 health), and the heavy chariot moves into take the city, destroying Yuris' archer with the capture. Two archers shoot at Yuris' eagle warrior to weaken it and (hopefully) prevent it from capturing the city back. Two archer shoot at Sullla's archer near Aquileia. I have 6 archers here, with another archer and heavy chariot coming from the south. Cologne has produced that archer, and it starts building a horseman now that I have my 2nd horse back.
Sullla has 3 legions and a horseman attacking Stockholm (you can see that at the northwest corner of the screenshot above). I'll have 4-5 turns to prepare for those troops if he decides to attack me from there, but there might be more troops near Aquileia, too.
Ulm finally finishes its granary (3/5 housing now) and it's at size 3 working the cattle, the truffles and the iron. Aachen puts down its own encampment since it's at size 7 now.
Here's the latest overall view. In terms of military, Sullla is at 446, I'm at 177 and Yuris is at 76. I'm about to finish Machinery, so I'm saving gold to upgrade at least a couple of the promoted archers to crossbowman and start producing more of those.