Cartographic evidence and the positions of its recorded town gates, to the east, south, and west, suggest the town once enclosed roughly 34 hectares, yet no town wall has survived, and much of that considerable area was given over not to buildings but to gardens, orchards, fields, and meadows. The dense urban core concentrated around a central crossroads, where a market cross stood and the parish church sat just to the south-east, while a great deal of what lay within the town boundary was cultivated open land.

The town was founded under the lordship of William Marshal, earl of Pembroke and lord of Leinster, who granted it a charter in 1217. It did not have an easy early history: it was burned in 1286 and again in 1317 during the Bruce invasion, on which occasion the town charter itself was destroyed in the fire. By 1344 Callan was substantial enough to be chosen as the site of a parliament, though that assembly never properly convened; the king, apparently alarmed that Maurice fitz Thomas Fitzgerald, Earl of Desmond, had arrived with several thousand armed men, issued an edict forbidding the magnates of Ireland from attending. In 1391 Hugh le Despenser sold the town, along with the advowson of its church and rents amounting to £33 15s 3d, to James Butler, Earl of Ormond, whose family would remain closely associated with it for centuries. The town sent two members to parliament from 1585 until the Act of Union in 1800, and in 1650 it was garrisoned by Ormond's forces with 1,500 men before surrendering to Cromwell's army after significant fighting.

The mid-seventeenth-century Civil Survey recorded 67 houses in the town, among them 31 thatched cabins, 16 thatched houses, and 20 slate-roofed buildings, as well as 20 plots described as containing only old stone walls, likely the shells of ruined structures. Two inns appear in the same survey: one on West Street called "The Sign of the Bell", and a larger establishment on Green Street named "The Sign of Helmet", described as a two-storey slate-roofed house with two chimneys, six rooms, and a cellar. Across the Kings River, a northern suburb, referred to in deeds from the fourteenth century onwards as "Callan le Hille", had its own streets, defences, and a town gate. Archaeological excavation has since turned up a medieval cesspit some 30 metres south of the West Street frontage, and a cobbled lane with a burgage boundary, the property division typical of medieval planned towns, on the south side of Mill Street, quiet confirmations that the town's layered past is still underfoot.