In 1381 Feb 10, Jonannes de Cobeham was granted, by Richard II, (In year 3 of his reign) a Royal licence to crenellate Coulyng (Cooling Castle)

"Licence for John de Cobeham to fortify and crenellate the mansion of his manor (mansum manerii sui) of Coulyng, co. Kent."

Granted at Westminster, by privy seal.

Cooling Castle was the seat of the de Cobham family after 1214 but the licence to crenellate was only granted in 1381, partly as a defence for the sea-access to London, commemorated on a copper plate in the east tower, formerly on the gate-house, which reads: 'Knouwyth that beth and schul be/ That I am mad in help of the cuntre/ In knowyng of whyche thyng/ Thys is chartre and wytnessyng.'

Original source is

• Calendar of Patent Rolls (1377-81) p596

(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;

• King, D.J.C., 1983, Castellarium Anglicanum (London: Kraus) Vol1 p240n11
  Turner, T.H. and Parker, J.H., 1859, Some account of Domestic Architecture in England (Oxford) Vol3 pt2 p303, 418

More details about licences to crenellate can be found here.

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