What would be some good alternate Capitals of the Eastern Roman empire? I imagine that a City controlling the
https://en.wikipedia.org/wiki/Dardanelles straight would do well. What do you guys think?
There are literally no better than Constantinople. It has list of advantages as long as my arm:
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Defensibility: city is located on a pennisula (itself on a pennisula), with good sheltered harbour, and strong currents which screw with attempts at navigation by anyone not familiar with them. It also means it is easy to resupply as long as control of the sea is maintained.
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Trade: very good harbour (Golden Horn), straddles a chokepoint: north-south trade all goes through Straits of Bosphorus, while East-West trade goes either through Danube-Black Sea (and thus close or through Constantinople) or overland through Thrace and Anatolia (and thus through Constantinople). Since strait is narrow, getting goods over it is not much of an issue.
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Administration: city is close to Sea of Marmara, Aegean Sea and Black Sea. Since sea communication is much easier over sea than over land, this is a major advantage - yet its location is not disadvantageous for overland communication either - see earlier section
Now let's look at alternatives proposed and where they fall short of Constantinople:
Nicaea: nowhere as defensible, not as good harbour, and in fact sits kinda aside from trade routes - even though its location in Sea of Marmara does provide it some advantages.
Perinthos: same as Nicaea.
Every other city: same as Nicaea, to greater or lesser extent. But I would say that Nicomedia is the best choice.
This does lead to the question if Classical, Hellenistic, and pre-Constantine Byzantium was actually a fairly thriving and populated city and we just don't hear of it much since the city-states down south during the Classical era, then the Hellenistic states post-Alexander, hogged up all the historical literature we do have as the serious power players of the time. Feels like in America as a modern-day example we sometimes forget how naturally vital and important certain cities like St. Louis, Chicago, or Cincinnati are in their gorgeous sites because all the attention's on the REALLY big metropolises like New York or Los Angeles.
From reading Greek history, I always got impression that even Classical Byzantium was a major city, at level with Athens perhaps, but was simply too far away from mainland Greece to be "interesting". Hence the lack of sources.
That being said, considering how important aquaducts were for Constantinople, it might be that lack of water that prevented its development prior to Roman times (there is a stream within Constantinople IIRC, but I don't know how large - likely fairly small, considering how important cisterns were for sieges).
