castle Coombe is one of my local circuits. I've driven it a few times, plus photographed there lots.
For parking, the main carparks are, coming in from the A420, at Quarry, the Paddock/Folly's and then further on you can park up from Camp to Tower.
It's worth walking around the circuit to get different shots. Obviously starting at the paddock area you can get all the static shots, get drivers to pose with their cars etc
You can only get around the outside of the circuit, but it's all walkable.
I doubt you'll get on the pit wall as you can on track days, but there's gaps in the fencing for panning shots along the start/finish straight, plus there are bumps on the inside at follys which the cars get a little unsettled over. It's not unknown for cars to spin off inside the circuit there (yes I've been that way too).
Walking clockwise, there's high banking around folly's for shots down into the cars, i.e.
but from folly's to avon rise you can't really get close enough to the track for good shots.
The cars go very light as you go over Avon rise and you have to get the braking right there, maybe a little squirt of power before throwing it around Quarry. because of this there's always incident at Quarry, especially on the first few laps. If you walk just past Quarry there's a dip between the banking for the emergency vehicle access and that's a great place to get the cars through quarry, i.e.
Further clockwise, you have the esses, which a good viewing area on top of the banking and there's always some challenges here, so again a good spot.
Further around you have Old paddock bend near the skid pan school and again that's a great place to get good long shots.
Occaisionally cars lose it here:
Hope this helps