The battle of Aljubarrota had been won with the help of British archers and, on 9 May 1386, Joao I, King of Portugal, concluded the treaty of Windsor with King Richard II of England. This stipulated that there should be 'an inviolable, eternal, solid, perpetual and true league of friendship, alliance and union' between the kingdoms.
It has never been violated and to this day Portugal is often described
as 'England's oldest ally'.
The alliance was further cemented by Joao's marriage to Philippa of Lancaster, daughter of John of Gaunt, and they became the parents of eight children. 'Dona Filipa', as she was known in Portugal, was to become the country's best loved Queen. Austere and pious by nature, she brought about a much needed reform in court life and she and the King brought up their large family with care.
Before his marriage, Joao I had fathered two illegitimate children but after their marriage he remained faithful to Philippa. On one occassion court gossip informed the Queen that he had strayed. He was able to convince her that this was not so and wryly commemorated the event by having one of the royal apartments decorated with a design of chattering magpies.

Source: Leo van de Pas