On 26 Mar 1350 [her grandfather] Alfonso "Avenger" XI King Castile (age 38) died of plague. His son [her father] Peter "Cruel" I King Castile (age 15) succeeded I King Castile.

On 03 Apr 1367 the forces of [her father] Peter "Cruel" I King Castile (age 32) commanded by Edward "Black Prince" (age 36) defeated the army of [her uncle] Henry "Fratricide" II King Castile (age 33) at the Battle of Nájera at Nájera. The English forces included John of Gaunt 1st Duke Lancaster (age 27), William Scrope, Peter Courtenay (age 21), John Devereux 1st Baron Devereux (age 30), John Savile of Shelley and Golcar (age 42), Bernard Brocas (age 37) and Thomas Banastre (age 33).

The forces of Henry "Fratricide" II King Castile (age 33) included Bertrand Geusclin (age 47). The battle achieved little since Bertrand Geusclin (age 47) escaped.

John Ferrers 4th Baron Ferrers of Chartley (age 36) was killed. His son Robert Ferrers 5th Baron Ferrers of Chartley (age 9) succeeded 5th Baron Ferrers of Chartley. Margaret Despencer Baroness Ferrers of Chartley by marriage Baroness Ferrers of Chartley.

Edward "Black Prince" (age 36) thereafter returned home, taking Peter's daughters [her illegitimate sister] Constance (age 13) and Isabella (age 12) as hostages to ensure Peter would pay the money owed to Edward. Constance (age 13) married John of Gaunt 1st Duke Lancaster (age 27), Isabella (age 12) married Edmund of Langley 1st Duke York (age 25).

On 23 Mar 1369 [her father] Peter "Cruel" I King Castile (age 34) was murdered by his half-brother [her uncle] Henry "Fratricide" II King Castile (age 35) outside the wall of the castle of Montiel, La Mancha. His half brother Henry "Fratricide" II King Castile (age 35) succeeded II King Castile.

On 23 Dec 1392 Isabella of Castile Duchess York (age 37) died. She the wife of Edmund of Langley 1st Duke York (age 51). Isabella had travelled to England with her sister [her illegitimate sister] Constance of Castile Duchess of Lancaster (age 38) who had married Edmund's elder brother John of Gaunt 1st Duke Lancaster (age 52). Isabella and Edmund's marriage was not, apparently, a happy one. She is known to have had an affair with John Holland 1st Duke Exeter (age 40) who may have been the father of [her son] Richard of Conisbrough 1st Earl Cambridge (age 7) progenitor of the House of York.