Church Wall
The high curtain wall behind you with its two small watch-towers at either end was built to protect the Cathedral from cannon fire. It would have had a raised platform for the soldiers to stand on.
Each bastion along the City Walls once had watch-towers. They were added in 1628 after the guards complained about having to do duty in the rain. These two on Church Wall are the only ones to survive.
In Church Wall is a sally port, or tunnel through the Walls, which the defenders used to make surprise attacks by rushing out and across the dry ditch. Hence the expression 'sally forth'.