Now a ruin standing on the eastern bank of the River Barrow, Carlow Castle is thought to have been built by William de Marshal (Earl of Pembroke and Lord of Leinster between 1207 and 1213) and is similar in design to Ferns Castle in County Wexford. At one time Carlow was an important and strong military fortress, strategically sited at the confluence of the rivers, and the castle withstood repeated attacks in 1494 and 1641.
Today, two battered towers and part of an intervening wall are all that remain after a local physician tried to remodel it as an asylum in 1814. In an effort to demolish the interior he placed explosive charges at its base and demolished all but the west wall and towers.