Impressive 15th Century fortress on the banks of the river Danube. It was once a fortified city and temporarily the capital of Serbia before it was overrun by forces of the Ottoman Empire but not until they had laid siege to the fortress on three separate occasions. 
The fortress survived the centuries more or less intact until bomb damage in World War II. In recent years the fortress has been restored back to it's former glory as has the spectacular fortress a few miles downriver in Golubac.
There isn't much else to detain you in Smederevo, it's an industrial city with a large steelworks that seems to coat everything in a red dust. From memory the nearest places of interest are Belgrade to the north and Veliko Gradiste to the south, both around 40 miles away.