I visited this site few months ago. Amazing medieval castle build by Despot Djuradj Brankovic in the begining of 15th century and its still standing. It was a last defence against Ottoman Turks. When they overrun Serbian army there, medieval Serbia stoped existing and then started a 500 years slavery under Ottomans. This is a reason why is so important site. Its very easy to come here. From Belgrade or any other city. You can see it coming into the city. Free to visit, except towers, but ticket is simbolic(around 100 dinars=less then 1 euro).