What remains is enough to read the original design clearly: a square artillery fort, thirty metres on each side, with salient angle-bastions projecting from each corner. Salient bastions are triangular or pointed outworks that extend beyond the main wall line, allowing defenders to fire along the flanks and eliminate the blind spots that plagued earlier medieval fortifications. The entrance was reached by a causeway midway along the eastern side, and the whole structure was ringed by an irregular fosse, essentially a defensive ditch. The north-east and south-east bastions survive more or less intact. The north-west has been disturbed by a nineteenth-century butt, a raised earthen mound likely used for target practice, and the entire south-western section has been steadily consumed by coastal erosion.

The fort was built in 1643, not, despite the popular name, by Cromwell's forces, but during a period of intense military activity in Ireland when controlling sea access to a town like Galway was of pressing strategic importance. It was one of a pair, constructed alongside a sister fort positioned near what is now Nimmo's Pier, on the opposite side of the harbour mouth. Together the two works were designed to command the sea approaches to Galway, subjecting any vessel attempting to enter the harbour to fire from both banks. The popular association with Cromwell, whose campaigns in Ireland belong to the latter part of the 1640s and into the 1650s, is a common kind of historical misattribution; in Irish folk memory, Cromwell tends to accumulate responsibility for a great deal of construction and destruction that predates him.