Fathom Castle, Medieval fortress in County Armagh, Northern Ireland
Fathom Castle was a stone fortress near Newry in County Armagh. The site lay strategically along the western approach to Carlingford and featured defensive walls that overlooked the surrounding countryside.
The fortress originated under Hugh DeLacy's control but was rebuilt and strengthened by Shane O'Neill in the 16th century. It remained under O'Neill hands until English forces took control.
The castle represented Norman influence in Ireland, marking the territory controlled by the Marmion family who maintained local governance from the 12th to 17th centuries.
The site was demolished in 1730 during canal construction, so no original structures remain visible today. Historical records and maps from the 17th century at the British Library document the original layout and location.
The site was known by several names including Feedom Castle and Fidun Castle, which derive from an Irish term meaning forest or wooded place. These varied names show how local terminology was recorded differently by different writers across the centuries.
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