In 1389 May 6, Abbas et Conventus Abbatiae de Thornton (Abbot Thomas Gresham) was granted, by Richard II, (In year 12 of his reign) a Royal licence to crenellate Abbatiae de Thornton (Thornton Abbey)

"quandam novam donum desuper et juxta portam Abbatiae suae de Thornton muro [the new house over and about the gate]"

Granted at Westminster, by privy seal.

Said to be a repeat of the 1382 licence, but possibly for new work at the abbey rather than a repeat. If so then the question is what was the reason that the previous licence did not cover the new work.
The suspicion must be that the abbot and abbey must have been in need of some royal favour because of something Gresham was doing.

Original source is

• Calendar of Patent Rolls (1389-92) p28

(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;

• Brooke, C.J., 2000, Safe Sanctuaries (Edinburgh; John Donald) p8
  Platt, C., 1990, The Architecture of Medieval Britain (New Haven and London: Yale University Press) p210
  Coulson, C., 1982, 'Hierarchism in Conventual Crenellation' Medieval Archaeology Vol26 p95
  'Houses of Austin canons: The abbey of Thornton', A History of the County of Lincoln: Volume 2 (1906), pp. 163-166. URL: http://www.british-history.ac.uk/report.aspx?compid=38019. Date accessed: 06 April 2008
  Turner, T.H. and Parker, J.H., 1859, Some account of Domestic Architecture in England (Oxford) Vol3 pt2 p231, 419, 420

Abbot Thomas Gresham

Abbot Thomas Gresham (sat 1364-94) 'a man of very evil life' (Cott. MS. Vesp. D. i, 40-7 sourced from VCH) Pages relating to him in a 16th century history of the abbey were torn out 200 years later – a scribbled note saying it “was to protect the abbey’s reputation”.

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