I suppose then that by the restoration century by the Komnenoi (end of 11th to end of 12th century) the Empire was mostly a nation. Although still less so than the successor states following 1204 (Epiros, Nicaea, Trebizond).
It should be noted that the Byz Empire was not a feudal state, unlike anything else in Europe. It had concrete laws and administrative mechanisms, which were what allowed it to resurrect following 1071 and then even after 1204.
I suppose that in the (much older) age of Heraclios (last Byzantine-Sassanid megawar, and rise of Islam) that also played an important part in the Empire not collapsing, despite the borders being at some point of the war reduced to mostly the Balkan provinces.