What made it difficult to attack Istanbul?

Attacking Istanbul, historically known as Constantinople, was challenging due to its formidable defensive walls, which had withstood numerous sieges over centuries. The city's strategic location on the Bosporus Strait allowed it to control maritime trade routes, and its natural geographical features, including hills and the surrounding water, provided additional defense. Additionally, the well-trained Byzantine military and their use of advanced artillery, like the famed Greek fire, made any assault particularly perilous.