Alonso de Arcos was the alcaide (Military Governor) of Tarifa.
He was a Reposteiro-Mór (a major footman at the royal household encharged with drawing and undrawing the curtains and hangings and treasurer of the store-house for furniture) of King Manuel I of Portugal and a Fidalgo of his Royal Household and Alcaide-Mór of Arraiolos.
In 1462, Alonso de Arcos, alcaide (Military Governor) of Tarifa, Rodrigo Ponce de León, son and heir of the Count of Arcos, and Juan Alonso de Guzmán, Duke of Medina Sidonia, attacked and gained control of the town of Gibraltar in the Eighth Siege of Gibraltar.
During the 1383–1385 Crisis, the alcaide (commander of the castle) count Viana sided with Castile.