Algiers was a city in Northern Africa.
The Doctor visited Algiers in 1516, where he was involved in what he referred to as "that terrible business with the Spaniards."
At a later time, the Fourth Doctor and Romana were brought to Algiers for interrogation by the Earth military. There they were interrogated by General Vincent. (AUDIO: The Sands of Life)
Behind the scenes
In real life, the 1516 event the Doctor was probably referring to was the Capture of Algiers, which was part of a conflict between Spain and the Ottoman Empire.