It was a viewing platform/tower at the top of the T&C fields. Like the one in the Park, but nowhere near as high.
The front of it was painted white, so that they could project some artsy stuff on it at night.
I think it was only ever there in 2016, maybe 2017 as well but definitely not the past few years.
I remember when watching bands in 98 I had to constantly be pulling one foot out of the mud otherwise i'd get stuck there and wouldn't be able to move at the end of the set.