A proportion of artillery, consisting of twenty-four pounders, with 1,500 rounds of ammunition per gun, six eight-inch howitzers, with 1,000, and four ten-inch mortars, with 500 rounds, and four sixty-eight-pounders, with a proportion of shells, were in ships at Los Passages; and with the army there were six eighteen-pounders: this quantity of artillery was deemed adequate to the attack of the place.
The town of St. Sebastian is built on a peninsula running nearly east and west; the northern side being washed by the river Urumea, the southern by the sea. The front defences, which cross the isthmus towards the land, are a double line of works, with the usual counterscarp, covered way, and glacis; but the works running lengthwise of the peninsula are only a single line, and, trusting to the water in their front to render them inaccessible, they are built without any cover; and the northern line is quite exposed, from the top to the bottom, to a range of hills on the right bank of the river, at a distance of six or seven hundred yards from it. These walls being uncovered, appears an unaccountable oversight, as the Urumea, for some hours before and after low water, is fordable, and the tide recedes so much, that for the same period there is a considerable dry space along the left bank of the river, by which troops can march to the foot of the wall.
Marshal Berwick, when he attacked St. Sebastian in 1719, aware of this circumstance, threw up batteries on those hills to break the town wall, and, whilst that was being effected, he pushed on approaches along the isthmus, and established himself on the covered way of the land front: as is but too frequently the case, as soon as the breach was practicable, the governor capitulated for the town, and the duke obliged him and the garrison to retire into the castle. It was now proposed to follow the same mode of attack, and as a preliminary, the garrison were to be driven from a post they occupied about seven or eight hundred yards in advance of the town, formed by the convent of St. Bartolomeo and a redoubt then in progress; and from a small circular work, which they made with casks on the causeway, four eighteen-pounders and two howitzers were put in battery for that purpose.
The operations against the town were commenced by the erection of batteries on the hills to the north of the Urumea, for twenty twenty-four-pounders, four eight-inch howitzers, four ten-inch mortars, and four sixty-eight-pounder carronades; the guns to breach the sea-wall between the two towers, the carronades to be used with shells only, and the mortars to be directed against the land front and castle.
On the 14th of July, the first two batteries opened on the convent of St. Bartolomeo.
15th of July.—A false attack was made on the convent of St. Bartolomeo, to ascertain if the enemy intended obstinately to defend it, which the troops carrying farther than was ordered, they were obliged to retire with some loss.
17th of July.—The end of the convent having been entirely beaten down, the ninth regiment and a Portuguese brigade assaulted and carried it with little difficulty.
Two more batteries for the eighteen-pounders and the two howitzers were thrown up in the night, in a situation to enfilade and take in reverse the defences of the town.
On the night of the 18th of July the suburbs of St. Martin, which the enemy had burned, were occupied: they, however, continued to hold the circular redoubt.
Night between the 19th and 20th of July.—Approaches were struck out to the right and left of St. Martin.