Question

what military tactic was used to capture constantinople

Answer 1

A massive siege followed by a huge assault. A lot of medieval warfare concerning fortified places like walled cities and castles were done using siegecraft -- surrounding the enemy, pummeling at them with siege weapons such as catapult, probing defenses with occasional attacks, and essentially trying to starve them into surrender. Nothing could get in, and neither could anything get back out.

Eventually the Ottomans decided to finally just up and attack the city, suffering massive losses but winning the battle.