The castle of San Lorenzo, which guarded the Chagres River's mouth, was built on the right bank of that river, on a high hill of great steepness. The hill has two peaks, with a sort of natural ditch some thirty feet in depth between them. The castle was built upon the seaward peak, and a narrow drawbridge crossed the gully to the other summit, which was barren and open to the sight. The river swept round the northern side of the hill with considerable force. To the south the hill was precipitous, and of such "infinite asperity," that no man could climb it. To the east was the bridged gully connecting the garrison with the isthmus. To the west, in a crook of the land, was the little port of Chagres, where ships might anchor in seven or eight fathoms, "being very fit for small vessels." Not far from the foot of the hill, facing the river's mouth, there was a battery of eight great guns commanding the approach. A little way beneath were two more batteries, each with six great guns, to supplement the one above. A path led from these lower batteries to the protected harbour. A steep flight of stairs, "hewed out of the rock," allowed the soldiers to pass from the water to the summit of the castle. The defences at the top of the hill were reinforced with palisadoes. The keep, or inner castle, was hedged about with a double fence of plank--the fences being six or seven feet apart, and the interstices filled in with earth, like gabions. On one side of the castle were the storesheds for merchandise and ammunition. On the other, and within the palisadoes everywhere, were soldiers' huts, built of mud and wattle, thatched with palm leaves, "after the manner of the Indians." Lastly, as a sort of outer defence, a great submerged rock prevented boats from coming too near the seaward side. Early in the morning Captain Bradly turned his hands up by the boatswain's pipe, and bade them breakfast off their beef and parched corn. Maize and charqui were packed into knapsacks for the march, and the pirates rowed ashore to open the campaign. The ruffians from Santa Katalina took their stations at the head of the leading company, with trusty pirates just behind them ready to pistol them if they played false. In good spirits they set forth from the beach, marching in the cool of the morning before the sun had risen. The way led through mangrove swamps, where the men sank to their knees in rotting grasses or plunged to their waists in slime. Those who have seen a tropical swamp will know how fierce the toil was. They were marching in a dank world belonging to an earlier age than ours. They were in the age of the coal strata, among wet, green things, in a silence only broken by the sound of dropping or by the bellow of an alligator. They were there in the filth, in the heat haze, in a mist of miasma and mosquitoes. In all probability they were swearing at themselves for coming thither.