Constantinople was at a strategic point trade because it was located

   It’s located on the route that connects mainland Europe with Asia. For example, the Pilgrims’ road, which led from Hungary to Palestine, passed through Constantinople.

   It controls the sea route that connects the Black Sea (it was from the coasts of this sea that large quantities of grain were imported, especially after the fall of Egypt) with the Mediterranean Sea. Abydos and Hieron, the two ports that guarded the straits of Hellespontus and Bosporus respectively, were under the direct control of Constantinople.

   It’s far closer to the Danube and the Euphrates frontiers than, say, Rome or most other important cities.

   There’s sea on three of its sides (the Sea of Marmara and the Golden Horn), so only a relatively small land area needed walls. That made the defense and supply much easier. Even during the Arab sieges, for example, the city was never completely cut off.

   Overall, it can easily control most of what J. Koder described as the three “core areas” of the Byzantine Empire: (a) the Aegean Sea, the Sea of Marmara and the first part of the Black Sea; (b) the western, northern and southern coast of Asia Minor; (c) Thrace, Peloponnese and the Aegean coasts of mainland Greece.

Explanation:

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