With the double-barreled idea of preventing any repetition of the Spanish occupation and of providing a securer harbor for his fleet, Khair-ed-Din conceived the idea of connecting the Penon with the city by means of a huge mole. For those days it was a herculean undertaking, but an ample supply of forced labor was at hand in the corsairs’ Christian captives, thirty thousand of whom were employed at the task, while an inexhaustible mine of building-materials was provided by the ruins of the old Roman city of Rusgania. The work was completed in three years, and thenceforward for more than three centuries the corsair fleets found refuge within, safe from the storms of the Mediterranean and the attacks of the enemies; for Khair-ed-Din mounted heavy batteries on the Penon and in 1544 erected a lofty lighthouse to guide his home-bound rovers. The present great harbor, covering 222 acres, was commenced by the French in 1836—the first time blocks of concrete were used in such an operation. From about 1518 until 1587, Algiers was the capital of the beylerbeys, the Turkish viceroys of North Africa, whose rule extended over Tripolitania, Tunisia, and Algeria. From 1587 until 1659 the Barbary states were governed by Turkish pashas, sent from Constantinople for terms of three years; but in 1659 a military revolt in Algiers put an end to this system of government, reduced the pashas to nonentities, and greatly weakened the Turkish power in Africa. From 1659 onward the Barbary states, though still nominally parts of the Ottoman Empire, were in fact anarchical pirate republics which chose their own rulers, supported themselves by plunder, and made their own treaties—which they rarely observed.