Rumeli Castle was one of the key defensive points during the conquest of Constantinople. It was built in 1452. It has an interesting story.
Sultan Mehmet II had a target; to conquer Constantinople.Ottoman Empire and East Roman Empire had a peace agreement in that period. Mehmet II had a clue to prevent the support of Byzantine alliens. A priest (he is claimed to be a Muslim indeed) pointed a place to Ottomans to assist their plans. Sultan Mehmet II asked for a hunting party to Byzantine Emperor. Ottoman Sultan made a plan to build a castle but he had to make Emperor accept it. Firstly emperor accepted sultan’s cottage. He thought to make a fait accompli. He made a deal with Byzantine Emperor. He would make plan for a cottage with leather line and Emperor led Sultan build in that border.
Mehmet II made a plan to build a castle and he did it in a few months. When the castle project finished, Byzantine emperor protested this attempt. Mehmet II told him that he obeyed the agreement so there was no conflict.