This is the quite aptly named Großes Fass (Great Barrel), and indeed it is! It is so big there is a staircase along the side and a platform across the top so you can walk across and around the entire thing! From this webpage:

The celebrity in the Castle is certainly the Great Barrel. The vaulted cellar to the left of Friedrich's Palace actually contains three barrels. The first was built in 1591 to hold 130,000 litres of wine. The second was built in 1664, held 195,000 litres and was guarded by the famous court dwarf Perkeo. The most celebrated, however, is the barrel built in 1751 (8.5 m large and 7 m high). With a capacity of more than 220,000 litres (58,000 US gallons) it is supposedly the world's greatest wine barrel and even has a dance-floor on top.

A pump once delivered the wine directly into the King's Hall in the Ladies Building. No wonder that allegedly 700,000 litres of wine were stored in the castle, as the drink-happy inhabitants are said to have consumed 2,000 litres of wine every day!

The BIG Barrel