King John's Castle dates from the early Norman era, perhaps as early as 1200. King John visited Naas in 1206. He visited again in 1210, when he held a form of Parliament in the town. About this time Kildare became a separate County. This assembly would appear to have been held in the newly built Naas Castle.

During the thirteenth and fourteenth centuries the town became a Norman stronghold. In 1409 Henry IV granted to Naas its first charter as a Corporation and a few years later it was given power to collect tolls at all the entrances to the town, the moneys to go towards fortifying the town with walls and gates. King John's Castle was rebuilt and incorporated into the town wall structure. The vaulted rooms of the old building still exist in the castle. There is a line of castles and houses to the north and east of Naas, which with its own defences, became the chief southern outpost of the "Pale" fence, ordered by "Poyning's" parliament in 1494.

King John's Castle is the last surviving example of the many fortified houses in the town of Naas, It is a large building of three stories, it comprises a tower, with a winding stone staircase, a dungeon, a Dining room on the ground floor, and an equally large Drawing room on the first floor. There is a variety of large and smaller rooms throughout the castle. An underground tunnel leads from the castle in the direction of the North Moat.