Yerm, I think you're quite smart and often agree with you tacitly. I've got to disagree with one of your points, while essentially backing everything else.
The location of Constantinople doesn't appear to be anything exceptional, defensively speaking. No more so than any other fortified coastal city. At the very least, it doesn't appear that the terrain hindered the Ottomans at all.
I'm assuming this is the result of a quick google-earth, or something. This is not correct
The geography is extraordinarily exceptional, which is one if the main reasons why Constantinople repulsed the first three Ottoman Sieges, three Rus sieges, the Bulgars, the Arabs, twice, the Persians, and Attila-the-freaking-Hun. Excepting the civil wars, which are hilariously complicated, the city was only ever breached on the two occasions attackers were let in - though the Ottomans were going to make it eventually.
Essentially, Constantinople lies on a tiny peninsula, created by the Golden Horn, that simultaneously renders naval assault preposterously complicated and creates a major choke point for land assault, which was then blocked by the Theodosian walls. It's also on hills, and the terrain is the sort of low grade upward that makes attack harder than it looks.
This is -not- like any fortified coastal city. The only good comps with major cities are Venice and the lagoons, and, like, a theoretical late-medieval NYC
The Theodosian walls, while impressive, wouldn't have remained the high point of defensive structures forever regardless. So either you'd give the defensiveness a timer or just simply represent it as a bastion, which I think would be a better solution for representing the walls. But if you just simply did that, the siege of Constantinople would last years, which isn't very representative of how things played out. Ultimately, EU4 is meant to trend towards historical outcomes, but leaving the possibility of alternatives to arise. Giving Constantinople a bastion without an associated heavy penalty to defensiveness means that you would never see remotely historical outcomes regarding the length of the siege. As does just increasing it's fort defensiveness.
Actually, representing it as a bastion would be kind of justified. This was the fourth Ottoman siege - the process did take years. But I also think a bastion is just too much. A unique +3 building is worth considering. In my opinion, it would be better to make a province modifier or unique "Golden Horn" terrain type, with something like +1 fort level, +5-10% defensiveness, however. (This way, something similar could be applied to Venice, etc.)
The fort level represents the force multiplier the choke point provides, the difficulty to fit enough heavy guns in, and the way-before-it's-time concentric defenses of the Theodosian Walls. (They would've had to change over the years, of course, but they're essentially either a concentric defense to, or a route already provided for, a hypothetical star-fort era structure.)
The defensiveness would represent the insane difficulty of blockading the place properly (it's a currents/local water issue) and the sea-side defensive works, which made starvation a slower process. I'd also need to check if the cisterns were running in 1444, which is why I said 5-10, and not just 5%.
Like I mentioned earlier, if it was just one of those things that lead to the fall of Constantinople (or if it took all of them), then I would agree that maybe it was just a bit of bad luck and not a problem of the lack of morale. But any one of them spelt the city's fall, and that they all happened pretty much at the same time indicates to me that the city wasn't going to hold too much longer.
There are some estimates that put it as high as 300K, but most, particularly the more modern analysis, put it considerably lower; in the 50-80K bracket. Which places it as not being very exception for an Ottoman siege even at 160K, given that Svetigrad and Krujë were sieged with an estimated 80K and 100-160K troops respectively.
The fall had been inevitable since at least 1341-1347 civil war, honestly. Basically, any alt-historical survival of the Basileia Rhomaion implies that turning out at least a bit differently, or the "empire" would start out bankrupt, etc.
There should be no Greek Fire modifier, because that wasn't very reliable or magical, anyway (it was most likely just crude oil, and couldn't be used unless the wind, weather, current, and opposing navy aligned perfectly). The notion that the 15th century imperial rump was about to undergo any Italy-like renaissance are laughable, honestly, and that's why there's an Eastern/Turkish tech group difference, anyway. EUIV deliberately starts after the Crusade of Varna precisely because Rhomania was terminal at that point - Constantinople was in a great location, but that's all the state had going for it, period.
