Captain Alexander Saunderson reportedly constructed the original fortification here, building a house of lime and stone along with a bawn, a defensive courtyard wall typical of Ulster plantation architecture, complete with two flankers for added protection. This initial structure met its demise during the Williamite wars of the late 1600s, when much of Ireland became a battlefield between the forces of the deposed Catholic King James II and the Protestant William of Orange.

The Castle Saunderson that stands today is quite different from its predecessor, being a composite structure that evolved over several centuries. The current building incorporates elements from the late 18th century, with additions and modifications made throughout the 19th century and into the early 20th century. This architectural patchwork tells the story of changing tastes, needs, and fortunes of its successive inhabitants over more than two hundred years.

While the original house is long gone, traces of the old bawn may still be discernible in the landscape. Archaeological evidence suggests that the defensive courtyard wall might have stood where the present courtyard lies, just west of the main house. This connection to the plantation period makes Castle Saunderson an important site for understanding the transformation of the Irish landscape during and after the Ulster Plantation, even if much of what visitors see today dates from considerably later periods.