Built in 1548 by Francis Bryan, Marshall of Ireland, on O'More lands, this imposing fortification was renamed Maryborough in 1556 to honour Queen Mary. The fort's original ground plan, preserved in maps from around 1560, reveals an ambitious rectangular enclosure measuring roughly 1,024 by 1,015 metres, complete with a circular tower at the northeast corner and a rectangular tower at the southwest. A single entrance pierced the western wall, whilst two storied buildings stretched 132 yards along the southern quadrant, all surrounded by an external moat that was partially water filled.

Life within these walls was far from peaceful. The garrison, which in 1565 consisted of a porter, drummer, ensign, surgeon and 39 arquebusiers under constable Francis Cosbie, faced repeated attacks. Orie M'Rory O Mordha burnt the town in 1597 and again the following year, whilst Cromwellian generals Hewson and Reynolds finally destroyed the fort in 1650. Today, fragments of the north, south and east walls survive, along with the circular angle tower that once stood three storeys high with an internal diameter of 8.2 metres. These sections now form unlikely boundaries; the northwest corner defines the Technical School grounds, portions along Church Street serve as garden walls for houses, and the eastern section has been incorporated into a modern flour mill.

Archaeological excavations in 2003 uncovered evidence of the original moat along Church Street, where a substantial cut was identified during broadband installation works. The defensive ditch, long since backfilled, left its mark on the urban landscape; property boundaries along Main Street still show a distinctive kink where landowners once extended their plots over the filled in fortification. Though much of the rectangular corner tower that Grose illustrated in the late eighteenth century was demolished around 1835, these surviving walls and archaeological traces continue to map the violent frontier between Gaelic and English worlds in Tudor Ireland.