It is impossible to do the Grosses Fass justice by a simple description alone. The only way to appreciate its magnificence is by seeing it in person. Allow me to quote from [the website]:
The Grosses Fass (or big barrel) is infact the biggest wooden barrel ever to have been filled with wine. The barrel was constructed in 1751 and can hold over 221,000 litres of wine! There's a little walkway which takes you across the top of the barrel (the top area was sometimes used for dancing).
The Grosses Fass is situated in 6 Heidelberg castle. The Colonel's Regiment were delighted to discover it during Mission 6 6 Heidelberg. They were initially duped by the Intermediate Fass which is situated in an ante-room outside the room housing the Grosses Fass. They paused for several minutes here, before walking around the corner and discovering that there was, in fact, a bigger Fass. Steve, David and Mike immediately went into shock (Rupert took it all in his stride). So amazed were they that they went back to see it a second time about an hour later. The Grosses Fass sparked the most amazing hour of comedy ever seen on a Colonel's break, as David, Mike and Steve reeled off application after application of "The Office" whilst Rupert looked on somewhat bemused.