In 1392 Sept 21, Johannes Devereux was granted, by Richard II, (In year 16 of his reign) a Royal licence to crenellate Penshurst (Penshurst Place)

"Licence for John Devereux to enclose his manor (manerium) of Penshurst, co. Kent, with a wall of stone and lime, crenellate and turret the same, and hold the premises in fee."

Granted at Woodstock Manor, by privy seal.

A licence to crenellate was granted in 1341. A second licence to crenellate, granted in 1392, produced an enclosing outer wall with side and angle towers.

Original source is

• Calendar of Patent Rolls (1392-96) p164

(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;

• Emery, Anthony, 2006, Greater Medieval Houses Vol3 (Cambridge) p440
  Turner, T.H. and Parker, J.H., 1859, Some account of Domestic Architecture in England (Oxford) Vol3 pt2 p420

More details about licences to crenellate can be found here.

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