It's the open area in front of the stage, there is some artificial grass to sit on. I would look at it an hour ahead, maybe get some food and claim your spot. I have seen the area very crowded, depending on the popularity of the artist.This year we plan on doing one of the concerts on Saturday. (all American rejects)
The concert starts at 8.30pm. How busy are these? how early do we need to arrive to get a "decent/OK spot". is 1/2hr prior enough time to get in to watch show?