The castle's history stretches back to the early 14th century when Henry Laffan, an official of the Butler Family, acquired 120 acres in Graystown from Gervase De Raley in 1305. By the time of the Civil Survey in 1654-6, the property was still in Laffan hands, with Henry Laffan of Graystown recorded as an 'Irish Papist' proprietor. The survey paints a picture of a substantial estate, describing "a good castle, a slate house wanting repaire with a large bawne & severall cabbins", whilst also noting some structures were in need of repair.

Built from roughly coursed limestone rubble, this imposing tower house measures 9 by 13 metres externally, with walls ranging from 0.8 to 2.16 metres thick. Its most striking feature is the gradual, very high base batter that rises 4 metres, giving the structure its characteristic defensive profile. The main entrance, a pointed doorway raised above ground level at the western end of the north wall, still shows fine architectural details including double chamfer and punch dressing on the jambs with drafted margins, plus yett-holes for securing the door. Though the south wall has largely collapsed and only corbels remain of the northwest corner machicolation, much of the internal layout remains legible.

The tower's interior reveals a complex arrangement of chambers, passages and mural rooms connected by a spiral staircase in the northeast angle. Each floor served different purposes; the ground floor featured wall cupboards and round-headed windows, whilst the first floor rested on wooden beams supported by paired corbels. The second floor boasted a fireplace and access to a mural chamber, topped by a pointed stone vault that also supported a garderobe chamber above. Three additional mural chambers built into the thick north wall provided extra living space, each with its own windows and some featuring wall cupboards. Despite centuries of weathering and the loss of its battlements and parts of the structure, Castle John remains an impressive example of medieval defensive architecture, its bones still telling the story of the Laffan family's prominence in Tipperary society.