Which feature was key to Constantinople’s becoming a large, powerful city?

Walls and water

Explanation:

Protected by walls and water, it commanded key trade routes linking Europe and Asia, making it Europe's busiest marketplace.

first, the romans stayed isolated because they had to defend their own territories from neighboring cities that were harassing them, like the etruscans. second, once the romans managed to defeat their neighbors, and started to expand their territory, they started to build roads and bridges. finally, now that the romans have covered their territory with roads functioning as highways, they could mobilize their legions faster to assemble larger military units in a specific place.