OK, I played for quite some time and I can share some experience. Despite the running time, the game remains completely stalemated until now, no city has fallen for all 4 parties.
1. As I said, there is a lot of land. Good land. It does need to be settled fast. Tight city placement also seems favorable in these settings, I regret not building another city in the vacuum E-SE of Croningen. Space needs to be grabbed. Settle all corners like I did and choose Haarlem and Harlingen wiser than I did (they will both need an aqueduct, which could have been avoided). The desert SW of the capital is an ideal founding site, getting rid of the desert square and exploiting the wheat fast to build settlers. Playing defensively around Rotterdam is the trick until you get the land.
2. We need to get the gems, Maastricht is in a very accurate position, allowing the gems to be shared with the allies. We need to get the spices fast and we need to settle approprietely to get the silks. For me on emperor, I am still trying to get the ivory close to Lisbon before culture traps them forever. I wonder if my city will be flipped towards my ally in these settings....
3. I fortify the hills south of Rotterdam, AI loves walking through them to the East and the rivered gold-iron hills must be protected from pillaging. This forces the AI to come against Rotterdam uncovered in the grasslands. A combination of med infandries, swiss mercs and trebuchets kept them in place until now. The two bonus grasslands must be kept safe from pillaging at all costs so swiss mercs can fortify there and protect injured med inf that just killed something there.
4. The Dutch have an advantage in the beggining, which I exploited by adding libraries to my cities. The swiss is very strong and offers a golden age. I am now in the point that the Germans get the advantage with those UU knights of his. I was in fact surprised to see them. I am holding properly until now. But during the swiss golden age, one must either hold defensively and grab land/build libraries or make a quick rush.
5. The Portug are holding until now. They are for all reasons that matter a part of our empire. Do not igonre them in a "ah, those guys jusst suck, I do not care" manner. I would not hesitate to send reinforcments if I spot they are in trouble. If they lose a city, it is very bad. If they conquer a city, it is very good. One may win the game without capturing a single city. They keep building lots of galleys unfortunately. It seems I cannot change techs with them. Or it just happened we discovered engineering together...
6. The river-crossing and the trebs from engineering helped of course, but perhaps a posteriori getting knights was better. I was hoping to trade with my allies. I will now ignore that stuff and head straight for cavarly.
7. I built libraries/aqueducts, but I never found time for marketplaces/courthouses, the Germans were pressing a lot. I was only now struck from the plague, which I have never faced before and I am not sure how it works. This is why I ordered marketplaces, perhaps troops from an infected city must not spread the disease? Like total war and other similar games? Not sure. I hope I will not be overrun with two less unit producing cities.
I liked this scenario quite a lot in general. How did you choose it? I will play it to the end.