Chichester city walls have been rebuilt many times since their Roman roots (the walls are believed to have been built from recycled flints from the burnt remains of the nearby Fishbourne Roman Palace), through its Georgian market town era and into the vibrant city we know today.
You will see a walkable section just behind GZ. If you look up and to the left of GZ there is a folly. The folly is itself began life as a bastion and if you walk through the tunnel that goes under the walls and into the adjoining carpark you can get up close to one of the few remaining bastions left along the city walls. If you have time, there is a circular walk around the city walls but don't forget to pay for your parking first!
If this is a quick cache and dash, use the carpark to the south of the wall as this is more easily accessible than finding your way into the city centre car park.
You are looking for a magnetic, recycled plastic tube!
Happy hunting