Beneath the lawns to the south of the castle, running roughly east to west, lies the ghost of a medieval route that once served as the main approach from the city toward Thomastown, passing through a barbican and the Castle Gate in the town wall before reaching the inner ward. A barbican was a fortified outer gateway, typically projecting forward from the main entrance to a walled town or castle, and this one formed part of the outer ward defences of Kilkenny Castle. The medieval road sat about 40 metres to the northeast of the current Castle Road, and its presence was confirmed in 2010 when a geophysical survey identified it as a clear east-west linear anomaly, roughly 60 metres long and 2 metres wide, its position matching almost exactly what the cartographer John Rocque had drawn on his 1758 map of Kilkenny.

The road's history can be traced in some detail. A grant dated 1435 from James, Earl of Ormond, to a Kilkenny man named William Boyd describes land lying between the Earl's garden wall and the common road used to travel from Boyd's property to the castle, confirming the route was well established by the fifteenth century at the latest. Francis Place, who sketched the castle from the east in 1699, and Rocque in 1758 both depicted it as a tree-lined avenue approaching the Inner Ward, giving it a formal, almost processional quality. The road fell out of use in 1769 when Walter Butler received permission to build a replacement route through the Castle Garden. The official justification was practical: the old road was described in the grant as being "much out of repair" and the new line would shorten the journey considerably. The Castle Gate in the town wall, which the medieval road had passed through, was demolished the same year to make way for the new arrangement. Physical traces of the old surface survived long enough to be noticed: paving was uncovered during trenching of the castle lawn sometime in the mid-nineteenth century, and further metalling was recorded during archaeological monitoring carried out by Ben Murtagh in 2000. A possible seventeenth-century fortification has also been identified on the north side of the road, just outside the castle moat.