Cat Fort occupied higher ground to the south-east of Elizabeth Fort, at around 34 metres above sea level compared to Elizabeth Fort's 18, precisely the kind of elevation that would let an enemy look straight down into the lower structure and render its defences useless. The solution was to build a smaller outwork up on that commanding ridge, plugging the tactical gap. It did not go well.

By the time the Williamite forces arrived in 1690, Cat Fort was described as a "new outwork, still unfinished", and it surrendered quickly, having no artillery with which to resist. The site's origins may reach back considerably further than its seventeenth-century military role, however. Scholars have speculated that the fortification was built on or adapted from an earlier ringfort, a roughly circular enclosed settlement of the early medieval period, possibly the "Lyscotekyn" recorded in documents from 1307. Phillips' map of 1685 places Cat Fort in the angle between Tower Street and Friar Street, which gives us a location if not a monument. Nothing of the structure survives above ground today.