I'm agreeing with most of what you post, but the last time I checked the Danube was quite a defenisible border? Main problem was when they couldn't even defend that one.And the geostrategic vulnerability is very much a fact; aside from the "natural fortress" of Anatolia - where the Empire indeed fell back to for quite a while - it was painfully short of "naturally defensible" borders, in particular in the dangerously immediate vicinity of the capital itself in the Balkans.
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