So opinion is a wall even a kilometre/mile well fortified and well armed would'nt have stopped the Ottoman. Quote: The city had about 20 km of walls (
Theodosian Walls: 5.5 km; sea walls along the
Golden Horn: 7 km; sea walls along the
Sea of Marmara: 7.5 km), one of the strongest sets of fortified walls in existence at the time. The walls had recently been repaired under
John VIII) and were in fairly good shape, giving the defenders sufficient reason to believe that they could hold out until help from the West arrived.
So if this sort of build was also present in the Chrysopolis and Chalcedon, it wouldn't have made a difference and Constantinople would have fallen as in OTL ?
The Theodosian Walls were effective, but incredibly expensive - doubling or tripling (or more) the construction work for this Mega-Constantinople is going to need some serious monies, and that'll hurt the Empire elsewhere.
And that wasn't even the issue - Constantinople at the end of the Empire wasn't the city of its past. It was perhaps the same size of Augsburg in Germany, and whilst it could muster troops, they couldn't fully man the entire wall, and the walls weren't designed with cannons in mind - to defend against, or defend WITH.
Now if it was done in stages, you could create impressive fortifications at Chrysopolis and Chalcedon - but they'd need outer fortresses to ensure naval security on the Bosphorus - as that would be the lifeblood of such a metropolis.
Constaintople, Galata, Chrysopolis and Chalcedon considered a single great city would be a terrifying force, but without complete, and utter control of the Sea of Marmara, or control over the Dardanelles to isolate the Sea of Marmara as a Greater Constantinople Lake - a strong naval force could isolate the different parts of the "City" and lead to urban disaster as they'd unintentionally become much more tightly bound - or have lapses in judgement regarding relying on Constantinople for stores, and troops, etc.
Using them all as a larger defense plan for the city is a good idea and I fully expect that contemporary defense plans included them all in some way - but simply put, using them to make a super city is a bad idea - because no other location in the area is as defensible, and I feel that being part of the City would cause complacency in the outer suburbs. (i.e. Chrysopolis & Chalcedon).
Now would those additional defenses make the region stronger. Yes. But each city NEEDS to think of itself as its own city, with its own stores, etc - rather than part of Constantinople, otherwise they'll rely on each other far too much to be self-sufficient in a worst case scenario. This would mean that Osman wouldn't have been able to take the cities as he did OTL with those levels of fortification, without utterly isolating the cities by sea first. But even then, they are strong bastions in their own right.
I would be intrigued to see the designs for these cities designed to rapidly accept friendly troops though, whilst sea-walled up to the back teeth.