A dry moat defined the northern line of the Castle defences.
The north was the only vulnerable side of the peninsula (its neck). It was also the side that any potential invaders were likely to approach the Castle. Therefore the moat, which lines the northern perimeter of the Castle, was a crucial line of defence.
While it is difficult to appreciate the dry moat today, the line it followed has been preserved in the form of Moatside Lane, whose tunnel-like nature is due to its original function.