The outer structure is pentagonal, a shape that was standard for serious artillery fortifications of the early modern period, with five earthen bastions at each angle and straight curtain walls between them enclosing an area of roughly 100 by 120 metres. At its centre sits a smaller, square inner fort, its walls pierced by gun loops and backed by earthen banks, with two towers set directly opposite each other and three gabled buildings arranged into a quadrangle. A covered way runs from the inner fort's north-east corner to a vaulted sally port, and from there continues north-east to a blockhouse on the waterline below.

The site has layers going back well before its current form. A tower called Castle Parks stood here and was considered of little military consequence when it appeared on a map in 1587. Spanish forces occupied and reinforced it during 1601, in the months surrounding the Battle of Kinsale, one of the most consequential engagements of the Nine Years' War. After that battle ended in defeat for the Irish and Spanish alliance, English crown engineers moved quickly: work on a new pentagonal bastioned fort, designed by Paul Ives, began in February 1602 and was finished in October 1604 at a total cost of £675. A bastioned fort, worth clarifying, is a design where angled projections at each corner eliminate blind spots and allow defenders to cover the full length of every wall with flanking fire. The structure was already requiring repairs by 1611, when an inner fort was also constructed. It was patched again in 1677 and then taken by Williamite forces in October 1690, during the broader campaign that followed the Jacobite wars. The landward bastions to the south-west are noticeably larger than the others, and some of the original stone facing survives on that side, along with the remains of a gatehouse and the revetment that once supported a drawbridge along the southern curtain wall.