A well-preserved round tower, standing to the NW of the Gothic cathedral dedicated to St Brigid. The lowest courses (up to approximately 2.9 m) are built of roughly dressed granite, thereafter the masonry is of coursed rubble, largely limestone. The battlements were added in the 1730s. The only sculpture is found around the doorway, which faces S, and is set 4.67 m above the ground.

Although the tower may be a reconstruction or replacement of an earlier tower, there is no reason to believe that the portal was added to the present structure. The portal is likely to date to the third quarter of the 12thc. Coins found below the stone floor of the tower in 1843 are likely to date from the 1140s at the earliest. Giraldus Cambrensis, c.1186, mentions a falcon that lived at the top of the tower.

Kildare is one of a small group of Irish round towers with Romanesque portals. The most ornate example is found eighteen miles away at Timahoe (Laois).