The siege of the Salamanca Forts (17-26 June 1812) saw Wellington detach a division from his army to eliminate the last French strong points in Salamanca, while the rest of his army continued to face Marmont.
The forts were actually converted convents, something that somewhat mislead Wellington, who had been expecting a fairly easy success. However the French had put a great deal of effort into making the forts more defensible. They had demolished the stone built colleges of the University that had surrounded the convents, and used the stone to strengthen the fortifications. They had also pulled down a large number of houses to give them a better field of fire.
The strongest of the three was San Vincente, on a steep hill at the south-west corner of the city. San Cayetano and La Merced, the smaller of the three, were located close together on another hill, separated from San Vincente by a ravine. The three forts could support each other. The only practical way to attack was from the north, hitting San Cayetano and San Vincente. Marmont left 800 men in the forts, with 36 guns - two in La Merced, four in San Cayetano and thirty in San Vincente.
As was so often the case, Wellington lacked a suitable siege train. Not having expected to deal with strong fortifications, he had only brought four 18-pounders, while another six 24-pounder howitzers were on their way. The heavier guns had been left behind at Cuidad Rodrigo and Almeida. The siege itself was left to the 6th Division, which had no experience of sieges.
Work on building a gun battery to the north of San Vincente began on the night of 17-18 June, but progress was limited. The battery was completed on the night of 18-19 June, and work began on two smaller ones. On the morning of 19 June the 18-pounders and three howitzers opened fire. They were able to bring down some of the upper parts of the fort, but couldn't reach the lower parts of the wall. Two howitzers were put in the second battery, but they came under heavy small arms fire, and twenty gunners were killed or wounded.
On 20 June Dickson arrived with the six 24-pounders. They were placed in the main battery, while the 18-pounders moved to the second battery. This was a more effective combination, but on 21 July the gunners almost ran out of ammo. Fresh stocks were summoned from Almeida, but they couldn't arrive until 26 July.
On the same day (20 June) Marmont's main army arrived in front of the British positions at San Cristobal, and for a couple of days it looked like a major battle might take place, but on the night of 22-23 June he withdrew. Wellington had called up a brigade from the 6th Division and the six 24-pounder howitzers just in case, but they were returned to Dickson on 23 June.
By now Dickson was running very short of ammo, with only 6 rounds for the 18-pounders and 160 for the six howitzers. The main effort was now made against the San Cayetano fort, but the entire stock of ammo was used up on the afternoon of 23 June without producing a breech. Even so Wellington ordered an attack to be launched that night. 300-400 men from the light companies of Bowes's and Hulse's brigades were to take part in the attack, but they had little confidence in its success. Only two of the twenty ladders allocated to the attack actually reached the walls of San Cayetano, having come under fire both from that fort and from Fort San Vicente. The attackers suffered 126 casualties, including General Bowes who was killed at the base of the walls.
On 24 June Marmont sent two divisions across to the south bank of the Tormes, in an attempt to outmanoeuvre Wellington. This forced Wellington to sent two divisions from San Cristobal to a new defensive position south of the river. The French briefly threatened to attack this new line, but then retreated back across the river.
On the morning of 26 June a convoy arrived from Almeida with 1,000 rounds, and that afternoon the bombardment resumed. The four 18-pounders were used to attack San Cayetano. Four of the howitzers were used to fire red hot shot onto the roof of San Vincente. The smaller guns were used to keep down the defenders. By the evening San Vincente had been set on fire in several places. The defenders put out the fires, but the British kept up the same tactics on 27 June, and more fires broke out. There was also now a practical breach in San Cayetano. Wellington judged that the time was right for another attack. This time he had a better starting point, as a trench had been dug along the base of the ravine between the two hills, but just before the attack was about to begin a white flag was raised at San Cayetano. The captain in command of that fort wanted a two hour truce in order to communicate with San Vincente, but promised to surrender at the end of the truce. Wellington gave him five minutes to surrender. He turned down those terms, and the assault began, but the defenders had effectively given up and the fort fell without much of a struggle - the attacking column only suffered six casualties.
At this point a white flag was raised above San Vincente, where the fires were now almost out of control. Once again Wellington demanded an immediate surrender, once again the French refused those terms and once again the place was stormed without any serious resistance, this time by the 9th Cacadores. A total of 600 unwounded prisoners were taken, while the French had suffered around 200 casualties. The British had suffered more heavily, mainly during the failed storm, with 99 dead and 331 wounded.
The fall of the forts allowed Wellington to concentrate his entire army against Marmont, and led to the series of manoeuvres that finally resulted in the battle of Salamanca.