Since Biz would be my next target, a few knights were sent to Biz' land to scout his army while my units were healing and waiting for the reinforcement.
A knight checked Essen: only 5 units. His main stack must be elsewhere
Maybe his main army was in the border city Hanover? But there were only 3 units
A knight scouted the area around Bis' capital. But all these cities only had 3 or 4 units:
Finally found his main stack in Munich - only these units???

I understood an AI who was never at war tended to have a limited army. But Biz ended a war with Toku not long ago, how could a 16-cities AI have such a tiny army

? I checked all the other German cities but didn't find any stack bigger than the one in Munich

.
After all, I needed a large army for the war against Biz, because he already adopted his favorite civic Nationhood. Even Biz' current army looked small, once the war started he would draft Grens like crazy throughout his large empire. The

showed Biz would get RP soon. He probably queued up Rifling.
The war against Biz was long. He indeed finished Rifling and drafted many units. Killed 30+ Rifles, 20+ Cavs, and 20+ Grens of Biz. Biz was the only enemy who got Rifling and MScience before the capitulation (AC also got Rifling but I was never at war with him, so I didn't consider AC an "enemy"). After losing more than half of his land, Biz finally capitulated.
T300 domination:
AC was cosplaying Pacal II in this game: he founded Hinduism then entered into full wonder-spamming mode

. When I was busy fighting Biz, AC already built UN and Hollywood

.
(And yes, as some people noticed, I captured SoL from Biz

)
The events at the end of game explained why Biz's army and techs were weird around T250 - T260. Biz lost two cities to Toku. Judging from the city name, Hanover was a late settlement; but Cologne was one of his earliest founded cities (the 3rd screenshot of this post showed Cologne was only 2 tiles from his capital). Losing Cologne definitely hurt Biz's economy.
As you can see in the screenshot below, two wars (me vs Alex; Toku vs Biz) almost broke out simultaneously.
The map also showed Toku aggressively settled on AC's island.
Biz managed to take his cities back with "the help" from Stalin, as Toku was in a two-fronts war. The recapture didn't show in the event log though, maybe because I didn't have Biz' map at that time. The event log in game couldn't show capturing a city which remained unknown to my map.
Capturing a city from Toku in Medieval era is often tough, unless the tech advantage is huge or there's a chance to catch Toku off guard. In Sulla's AI Survival game 6, AI Mehmed lost more than 100 Rifles and Grens but couldn't capture Toku's city (Kaitzilla wrote a detailed analysis
here ). Mehmed had Rifling and Grens in the AI survival game 6; but in this game Biz barely got Gunpowder when Toku attacked him. Imagine how many units Biz whipped, built, and lost for taking back the two lost cities. The war against Toku bled Biz dry. That was why Biz fell behind in techs and only had a small army around T257.
Isolation is not suggested when people move up to a new difficulty level, but isolation games can be interesting and educational for those who want to practise the empire management and naval warfare at a difficulty level they're used to. The leader is a strong one for people's first isolation attempt as well: CRE and Agri + Fishing start ensure a fast growth and strong early game; the Dutch UUs and UBs become strong in the mid and late game. The global land shape is quite forgiving: if some new players don't handle the isolation very well and fall behind, they can still find a weak target (the isolated Alex) to attack.
Thanks for the map