This rectangular tower house, measuring eight metres north to south and seven metres east to west, rises four storeys with its distinctive base batter and well-preserved quoins. One of these corner stones bears an inscribed date of 1501, marking the castle's construction during the Tudor period. The original entrance on the south side has been replaced over time, but the spiral staircase at the southeast corner remains, complete with elegant ogee-headed windows that speak to the craftsmanship of its medieval builders.

The castle's history is intertwined with the fortunes of the O'Kelly clan, who lost control of Moyvannan to Irish rebels by 1596. Evidence of the Kelly family's continued presence in the area can be found in nearby Kiltoom graveyard, about a kilometre north, where a graveslab commemorates William Kelly of Moyvenen. The tower's western wall retains two original double-light rectangular windows and three single rectangular openings, offering glimpses into its defensive past whilst flooding the interior with natural light.

During the 18th century, the castle underwent significant modifications that reflect changing architectural tastes and religious practices. A chapel was added to the north side of the tower, and pointed Gothic Revival doorways and windows were inserted into the eastern wall and throughout the structure. The current parapet likely dates from this period of renovation as well. In the 1980s, both the tower and the former chapel were renovated for use as dwellings, ensuring that this remarkable piece of Roscommon's heritage continues to serve a practical purpose whilst preserving its historical character for future generations.