In the 1220s St John's Tower was constructed in the moat to allow defenders to shoot at an attacking force.

In the Napolionic wars (1804) a gallery was built accross the moat which could house canons and riflemen who could protect the moat.

The end of the gallery can be seen in the photo.
The stairs lead down to a tunnel that passes under the gallery and on to a guardroom outside the moat.