One of the battles of the Hundred Ýears’ War was fought in Portugal on the 14th August, 1385, on the so-called ‘Cumeira de Aljubarrota’, about 80 miles north of Lisbon. There, the small army of King D. Joao I, led by Constable D. Nuno Àlvares Pereira, faced the greater forces of D. Juan I of Castile. Under the Portuguese flag, as Professor Peter Russell has shown, about 700 English archers fought, and on the enemy's side were many subjects of the King of France (n. 1). There are different contemporary versions of the battle which have caused a certain perplexity among historians. Froissart's chronicle tells us that D. Nuno Àlvares Pereira's men had fortified the battlefield; this assertion figures in another narrative known as the Cronica del Despensero, and also in the well-known letter written by D. Juan I of Castile, dated the 29th August, 1385. The Castilian Chronicler, Ayala, an eye-witness of the battle, says nothing about it however, and this fact must have prompted another chronicler, Fernão Lopes, who wrote 60 years after the battle, to assert that there was nothing to be said about the defensive system, even giving us to understand that it never existed. This conflict in the evidence had made it impossible to make a thorough investigation of a battle which is fundamentally important to Portuguese history. To overcome this, in February, 1960, the Ministry of War ordered the Commission of Military History to submit the battle to an intensive study and to make an attempt to evaluate the contradictory versions.