Cadiz, in the days when the practicable range of the heaviest artillery did not exceed 2,500 yards, was one of the strongest places in the world. The town lies on the extreme point of a long sandy peninsula, which runs out into the sea from the Isla de Leon, a large island separated from the mainland of Andalusia by the salt-water channel of the Rio Santi Petri, an arm of the sea varying from 300 to 400 yards in breadth, and flowing through marshes which make access to its banks very difficult. The Isla, protected by this enormous wet ditch, has a front towards the continent of about seven miles, from the naval arsenal of La Carraca at its north end to the Castle of Santi Petri at its south. Batteries had already been thrown up at all the commanding points, and Albuquerque had broken the only bridge, that of Zuazo, which crossed the marsh and the Rio. It would be impossible to pass the channel save by collecting great quantities of boats, and these would have to move under artillery fire. Venegas, the military governor of Cadiz, had already ordered all the vessels, small and great, of the villages round the bay to be destroyed or brought across to the city. Moreover, there were a score of gunboats in the channel, manned from the Spanish fleet, which could be used to oppose any attempt to cross the Rio. Indeed, naval assistance to any amount was available for the defence of Cadiz: there were a dozen Spanish and four English line-of-battle ships in the harbour. All through the three long years while the French lay in front of the Isla, no attempt was ever made to throw a force in boats across the channel: the venture seemed too hazardous. If, however, Victor had, by some expedient, succeeded in crossing the Rio, there were two lines of defence behind it, of far greater strength than that formed by this outer ditch of the Cadiz works. The triangular Isla de Leon forms with its apex a long sand-spit, which projects for four miles into the Atlantic. Half way along it the breadth of the spit is contracted to no more than 200 yards, and here there was a continuous entrenchment from water to water, called the Cortadura, or the battery of San Fernando, armed with many heavy guns. Supposing this isthmus to have been passed, there lies, two miles further along the sand-spit, the outer enceinte of Cadiz itself, with a front of not more than 400 yards in breadth, and deep water on either side.