Answer:
One advantage that the Christians inside the city of Constantinople had that presented obstacles to Mehmed II's invasion was the city's strong fortifications, which included thick walls, towers, and a moat. The defenders were able to use these defenses to hold off the Ottoman siege for weeks, even though they were vastly outnumbered and outgunned. Additionally, the city's location on a peninsula made it difficult for the Ottoman navy to blockade the harbor and prevent supplies from reaching the city. These factors contributed to the city's ability to resist the Ottoman invasion for as long as it did.