The portcullis groove is still visible in the passage, the chamfered limestone jambs of pointed arches still frame doorways that once controlled access to one of the most strategically loaded structures in the city, and a barrel vault that was originally shaped using wattle centring, a technique where woven branches formed the temporary mould for wet mortar, has largely been replaced by a modern flat ceiling without anyone making much of a fuss about it. It is the kind of place where centuries of adaptation have left the original fabric present but well disguised.

The citadel was built on Limerick's medieval town defences, and its origins are tied to a specific and often-misread moment in Irish history. According to historian Hodkinson, writing in 2006, the structure is Cromwellian in origin, raised in the immediate aftermath of the 1651 siege not to keep enemies out but to keep the newly subjugated town in check. Its form reflects this intent: a rectangle with two diamond-shaped bastions projecting at the corners that faced inward, towards the city itself. The citadel is also mentioned in connection with the Confederate period by Harold Leask in his 1941 analysis, suggesting the building's history may be slightly more layered than a single construction phase. By 1690 it had been refortified, and Story's map of the Williamite siege of that year records two star-shaped forts to the east of the city. A fragment of the seventeenth-century curtain wall survives some 20 metres north of the gatehouse, an L-shaped run of roughly coursed limestone standing up to 3.3 metres high and forming part of what was the western bastion.

The gatehouse itself is incorporated into St. John's Hospital, which means public access to the interior is not straightforward. The surviving section of citadel wall to the north is more readily observable from the street and repays a close look: the slight external batter of the masonry, the dressed limestone jambs of the archway cutting through it, and the lower skin of what appears to be a later wall added against its outer face are all legible if you know what you are looking for. The area around St. John's Square, just to the south, still follows the outline of the old defences, and read against a historical map, the remnants of this mid-seventeenth-century fortification start to make surprising spatial sense within the modern streetscape.