In 1322 June 23, W. Episcopus Exon (Bishop Walter Stapledon) was granted, by Edward II, (In year 15 of his reign) a Royal licence to crenellate Exon (Exeter Episcopal Palace and Cathedral Close)

"... clausum et mansum ... palatii sui episcopatus Exon., in Exon."

Granted at York, by King.

Licences to crenellate granted to Bishop Peter Quinel in 1290 and Bishop Walter Stapledon in 1322. Some slight remains of the cathedral close and palace buildings survive in later buildings.

Original source is

• Calendar of Patent Rolls (1321-24) p140

(In fact, the original source given is usually a transcription/translation of what are precious medieval documents not readily availably. It should be noted that these transcription/translations often date to the nineteenth or early twentieth centuries and that unwitting bias of transcribers may affect the translation. Care should also be taken to avoid giving modern meaning to the medieval use of certain stock words and terms.)

Significant later source are;

• Thompson, M.W., 1998, Medieval bishops' houses in England and Wales (Aldershot: Ashgate Publishing) p167
  Higham, R. A., 1988, 'Devon Castles: an annotated list' Proceedings of the Devon Archaeological Society Vol46 p146
  Coulson, C., 1982, 'Hierarchism in Conventual Crenellation' Medieval Archaeology Vol26 p93
  Turner, T.H. and Parker, J.H., 1859, Some account of Domestic Architecture in England (Oxford) Vol3 pt2 p408

More details about licences to crenellate can be found here.

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