Complex sentence: The Ottomans unsuccessfully besieged it in 1412,1427 and in 1439, when the Ottomans besieged both Smederevo and Novo Brdo.
Split into simpler sentences: The Ottomans unsuccessfully besieged it in 1412 and 1427. In 1439 the Ottomans besieged both Smederevo and Novo Brdo.