In 1361 July 1, Prior et Conventus de Lewes was granted, by Edward III, (In year 34 of his reign) a Royal licence to crenellate Lewes (Lewes Priory)

"Prioratum ac ecclesiam et domos ejusdem Prioratus"

Granted at Westminster, by King.

A response to French raids? a response to the Black Death? an increase of available funds after the Black Death?

Original source is

• Calendar of Patent Rolls (1358-61) p444

(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;

• Coulson, C., 1982, 'Hierarchism in Conventual Crenellation' Medieval Archaeology Vol26 p94
  Turner, T.H. and Parker, J.H., 1859, Some account of Domestic Architecture in England (Oxford) Vol3 pt2 p416

More details about licences to crenellate can be found here.

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