June a British party stormed a detached redoubt on the La Cabana heights. Only then did the British command realize how strong the Morro was, surrounded by brushwood and protected by a large ditch. With the arrival of their siege train the next day, the British began erecting batteries among the trees on La Cabana hill overlooking the Morro (some higher) as well as the city and the bay. Surprisingly, this hill had been left undefended by the Spanish army despite its well-known strategic importance. Charles III of Spain had earlier instructed Prado to fortify this hill, a task that he considered the most urgent of those relating to the fortifications. The task had been started but no guns had been installed. Two days later a British detachment landed at Torreón de la Chorrera, on the west side of the harbour. Meanwhile, Colonel Patrick Mackellar, an engineer, was overseeing the construction of the siegeworks against the Morro. Since digging trenches was impossible, he resolved to erect breastworks instead. He planned to mine towards a bastion of the Morro once his siege works would have reached the ditch and to create a runway across this ditch with the rubble produced by his mining activities. By 22 June, four British batteries totalling twelve heavy guns and 38 mortars opened fire on the Morro from La Cabana. Mackellar gradually advanced his breastworks towards the ditch under cover of these batteries so by the end of the month the British had increased their daily direct hits on the Morro to 500. Velasco was losing as many as 30 men each day, and the workload of repairing the fortress every night was so exhausting that men had to be rotated into the fort from the city every three days. Velasco finally managed to convince Prado that a raid was necessary against the British batteries. At dawn on 29 June 988 men (a mixed company of grenadiers, marines, engineers, and slaves) attacked the siege works. They reached the British batteries from the rear and started to spike guns, but British reaction was swift, and the attackers were repulsed before they caused any serious damage. On 1 July, the British launched a combined land and naval attack on the Morro. The fleet detached four ships of the line for this purpose: HMS Stirling Castle, HMS Dragon, HMS Marlborough and HMS Cambridge. The naval and land artillery simultaneously