In 1402 Oct 13, Johannes Corp was granted, by Henry IV, (In year 4 of his reign) a Royal licence to crenellate Dertemuth (Gommerock)

"Licence for John Corp of Dertemuth to crenellate a lodging his by the entrance of the port of the town for defence against the kings enemies.
quaddam hospitium juxta introitum portus vill de Dertemoth"

Granted at Westminster, by privy seal.

Licence for defence against the king's enemies. Clearly intended to be a serious fortification. Called a municipal licence by Coulson. Sometimes identified as an earlier building on or near the site of Kingswear castle, but the Gommerock identification seems more likely to me.

Original source is

• Calendar of Patent Rolls (1402-5) p219

(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., 1995, 'Battlements and the Bourgeoisie: Municipal Status and the Apparatus of Urban Defence' in Church, Stephen (ed), Medieval Knighthood Vol. 5 (Boydell) p170n201, 189
  Higham, R. A., 1988, 'Devon Castles: an annotated list' Proceedings of the Devon Archaeological Society Vol46 p146
  Turner, T.H. and Parker, J.H., 1859, Some account of Domestic Architecture in England (Oxford) Vol3 pt2 p421[wrongly identified as Dartmouth Castle]

More details about licences to crenellate can be found here.

Please let me know if you see any errors, can add information or can otherwise help to improve this resource. Contact me