From the Wikipedia page 
Sir James Blount (died 1493) (sometimes spelt Blunt) was commander of the English fortress of Hammes, near Calais. When in 1484 the Earl of Oxford was imprisoned there, Blount was apparently persuaded to switch to the Lancastrian side. Blount and Oxford fled to join Henry Tudor (the future Henry VII of England), leaving his wife in charge. She and the garrison held out for months against Richard III's forces, until in early 1485 they surrendered in return for safe passage into France.