There has been a castle on this site since 1172 when Norman knight Richard de Clare, known as Strongbow, built a wooden tower overlooking the River Nore. The first stone castle, much of which survives today, was built some 20 years later by William Marshal, Earl of Pembroke.

This medieval castle was roughly square in plan, with the main twin-towered entrance gate located in the now missing east wall. Kilkenny Castle is one of the few castles of its era built without a Keep.