The Down Survey maps, created between 1655 and 1656, clearly mark a castle somewhere in the vicinity, though its exact location remains frustratingly unclear. These surveys, commissioned by Oliver Cromwell to redistribute Irish lands to his soldiers and adventurers, provide one of the earliest systematic mappings of Ireland, making this reference particularly significant for understanding the area's medieval landscape.

The castle at Seapoint, also known as Templehill, likely served as a defensive stronghold overlooking Dublin Bay during the medieval period. Its strategic position would have allowed defenders to monitor maritime traffic and protect the coastline from raiders. Like many Irish castles, it probably began as a timber fortification before being rebuilt in stone, possibly by Anglo-Norman settlers who arrived in the area during the 12th and 13th centuries.

Today, no visible traces of the castle remain above ground, leaving historians and archaeologists to piece together its story from documentary evidence alone. The ambiguity surrounding its precise location has sparked debate among local historians; some suggest it may have been demolished during the construction of the railway in the 1830s, whilst others believe it could have been incorporated into later buildings in the area. This lost castle serves as a reminder of how much of Ireland's medieval heritage has vanished from the landscape, surviving only in old maps and written records compiled by researchers like Geraldine Stout and Padraig Clancy.