The blockade was maintained in three grand divisions or entrenched positions, namely, Chiclana, Puerto Real, and Santa Maria. The first, having its left on the sea coast near the Torre Bermeja, was from thence carried across the Almanza, and the Chiclana rivers, to the Zuraque, on a line of eight miles, traced along a range of thickly wooded hills, and bordering a marsh from one to three miles broad. This marsh, traversed in its breadth by the above-mentioned rivers, and by a number of navigable water courses or creeks, was also cut in its whole length by the Santi Petri, a natural channel connecting the upper harbour of Cadiz with the open sea. The Santi Petri, nine miles long, from two to three hundred yards wide, and of depth to float a seventy-four, received the waters of all the creeks crossing the marsh and was the first Spanish line of defence. In the centre, the bridge of Zuazo, by which the only road to Cadiz passes, was broken and defended by batteries on both sides. On the right hand, the Caraccas, or Royal Arsenal, situated on an island just in the harbour mouth of the channel, and on account of the marsh inattackable, save by water or by bombardment, was covered with strong batteries and served as an advanced post. On the left hand the castle of Santi Petri, also built on an island, defended the sea mouth of the channel. Beyond the Santi Petri was the Isla de Leon, in form a triangular island, the base of which rests on that channel, the right side on the harbour, the left on the open sea, and the apex points towards Cadiz. All this island was a salt-marsh, except one high and strong ridge in the centre, about four miles long, upon which the large town of La Isla stands, and which being within cannon shot of the Santi Petri, offered the second line of defence. From the apex, called the Torre Gardo, a low and narrow isthmus about five miles long, connected the island with the rocks upon which Cadiz stood, and across the centre of this narrow isthmus, a cut called the Cortadura, defended by the large unfinished fort of Fernando, offered a third line of defence. The fourth and final line, was the land front of the city itself, regularly and completely fortified. On the Chiclana side therefore, the hostile forces were only separated by the marsh; and although the Spaniards commanded the Santi Petri, the French having their chief depôts in the town of Chiclana, could always acquire the mastery in the marsh and might force the passage of the channel, because the Chiclana, Zuraque, and Almanza creeks, were navigable above the lines of contravallation. The thick woods behind, also afforded the means of constructing an armed flotilla, and such was the nature of the ground bordering the Santi Petri itself, on both sides, that off the high road, it could only be approached by water, or by narrow footpaths, leading between the salt-pans of the marsh.