This was one of the original gates and provided direct access from the city to the river side where ships would dock. In the map shown on page 5 it has a tall watch tower built over it but that has long gone.
At a corner of the wall beside Saint Columbs Cathedral, this was at the south end of the outside redoubt seen on the left side of the page 5 map. A small sentry tower still stands here but it seems in poor condition. This area would have been heavily attacked during the siege as the ground outside is relatively gently sloped.
St Columbs Cathedral.
This was the first post-Reformation Cathedral built in the British Isles and completed in 1633. During the great Siege cannon were hauled up onto the roof and used to fire upon the attacking troops as they approached the walls. They would have been firing over the Church Bastion.