In upper Castle St. you will pass a stretch of city wall with small cannon openings which can be clearly seen on the inner face of the wall. The wall still retains the remains of the wall-walk which would have projected out from the top of the wall.

The Double Tower is so-called probably because the interior consists of two chambers, one of which comprises a passageway providing access to the Benedictine Priory; the other larger chamber housed a stairway for access to the upper floors and battlements. Although the Church of St. John's priory was within the walled city, the priory's lands and its chief buildings were located outside the walls. During the Cromwellian siege of the city in 1649 this section of the city walls was pounded by cannon.

These towers were not built with cannon in mind and therefore had to be strengthened. The Double Tower was half filled with earth to help take the impact of the cannon. Continuing up Castle St., the section of wall that projects outwards is known as the "ramparts".

The "ramparts" belong to the age of gunpowder and were seen as a platform for heavy guns.

Waterford became the only Irish city that Oliver Cromwell failed to take. However, in the following summer, demoralised by blockade, plague and famine, the city surrendered to his son-in-law, General Henry Ireton.