Between 1611 and 1613, he constructed a brick and lime fortress atop this prehistoric earthwork, blending centuries of Irish history into a single site. Today, visitors can still spot traces of Fishe's ambitions in the form of a subrectangular hollow measuring roughly 18 metres from northeast to southwest and just over a metre from northwest to southeast, where loose masonry hints at the building's former presence.

Archaeological investigations have revealed intriguing possibilities about the castle's structure. In 1948, historian Davies suggested that the visible depression might mark the location of the house's cellars, offering a tantalising glimpse into how the building was organised. The choice to build within an existing rath was not uncommon during this period; landowners often repurposed these ancient earthen enclosures, which had served as fortified homesteads for Gaelic nobility centuries earlier, as ready-made defensive platforms for their own residences.

The castle's story came to an end in 1719 when it was abandoned, leaving behind only these fragmentary remains. The site represents a fascinating layer cake of Irish history, where a Bronze Age or Early Medieval rath became the foundation for a 17th-century plantation castle, each era leaving its mark on the landscape. Researchers including O'Reilly and Wilsdon have documented the site's evolution, ensuring that even as the physical structure crumbles, its story remains part of Cavan's rich archaeological record.