That's the amphitheatre in Finsbury Park. It has steps, rocks and seating steps as you can see.
I'm going to take a broom along next time and sweep it as there is debris about at the moment. It's close to the central cafe and loos to it's a good spot. There is a big path around the whole part that I intend to start walking around once I'm confident enough.
Knowing my luck i'll see some hot chicks and do a mean splits or fall dog mess...
Finsbury Park is London N4 on the Victoria and Piccadilly Lines as well as National Rail.
Just 2 stops from Kings Cross on the Victoria Line.