[thriftwy]

London, Rome and Athens don't strike me as seaports. They're somewhere not far from the sea, but not on the coast.

Venice reinforces my point, if anything. It was more like a large corporation than an empire, and then it failed to produce a nation state.

Istanbul is not a capital of Turkey. Before that it was a city-state of Constantinople for a long time.

[Piskvorrr]

London used to be the seaport of England (before railways outsourced this to Southampton); whence do you consider the name Docklands (the huge area downriver of the Tower)? The rest is just nitpicking ("modern Piraeus is not part of Athens") and moving the goalposts (first it's "historic capitals", now it doesn't matter because Istanbul is not a de-iure capital at present, even though it de-facto is? Plus, there's a major stretch of history between Constantinople and present day)