After two days of trench warfare, Carlist troops poured heavy artillery fire on the Liberal lines and launched an all-out offensive, supported by reinforcements brought in from Navarre by José Ignacio de Uranga.

Only two British Auxiliary Legion regiments and a number of their Basque guides, the Chapelgorris, were left to resist the Carlist advance, but were eventually overrun and captured.

The battle led to the end of the British Auxiliary Legion as an effective fighting force, with two-thirds of their members killed, wounded or executed by the Carlists or local civilians.

[1] The British Auxiliary Legion, which had been defeated at Oriamendi and was based at San Sebastián, took advantage of the circumstances to lead a Liberal offensive along the Cantabrian coast, only defended by isolated garrisons.

[2][3] When the Carlist troops were approaching Madrid, Spanish Liberal General O'Donnell left the besieged San Sebastián to launch a successful offensive to the south, on Hernani and Urnieta.

[4] The Liberals pushed their enemies towards the natural barrier formed by the rivers Oria and Leizaran, where the two armies stood facing each other at a distance of a mere 200 yards.

The Carlist bunker was connected with their rearguard by a ravine which edges were protected by rocky crags, while the gaps were closed with casket filled with clay and stones.

On the right side of the trenches, the terrain allowed O'Donnell to command the enemies' positions, which were occasionally checked by an artillery battery of British Auxiliary Legion, which inflicted a number of casualties among the Carlists when their rounds struck home.

[7] During the buildup process, Navarrese troops led by General José Ignacio de Uranga, departing from Arróniz in Navarre,[8] sneaked through the valley between Tolosa and Andoain.

The exchange lasted until 11 o'clock, when the rebel infantry emerged from their entrenchments, and supported by intense musketry fire, advanced on the left and right flanks of the Liberal army.

[5] Uranga's plan was to lure the main body of the Liberal army to their right, north of Andoain, and then launch a two-pronged attack on the weakened center and left.

The Chapelgorris were forced to abandon their posts at the bridge after fierce fighting, as the right wing of the Carlist attack made gains on the eastern bank of the river.

[5] According to British sources, it was at this stage of the battle that Colonel F. R. Clarke, in charge of a Scottish unit, mustered a column of nearly 300 troops in the town's main square and launched a bayonet attack in the direction of the bridge, which eventually drove the Carlists back to the river's edge.

The thick walls of the building provided a good protection against gunfire, and in its stores had plenty of food and supplies, but the 25 soldiers eventually surrendered to the Carlists on 16 September, on the promise of mercy.

It was at this stage of the battle that a company of Lancers carried out a rearguard action, in an attempt to relieve the Scottish units and Rifle Brigade troops surrounded in the village and recover a cart of rockets overturned in the retreat.

[4] All British Auxiliary Legion members captured at Andoain were forced to march to the rebel headquarters of Tolosa, where they were likewise massacred on the main square.

[6] British sources from Pamplona reported instead that the prisoners of war from the Legion who were captured in Andoain were killed in situ, with an exception made for 37 soldiers, 20 of whom were stabbed to death on the road to Tolosa and the remainding 17 executed by firing squad in the Carlist stronghold.

[18] Legion surgeon Henry Wilkinson and the British press of the time differ slightly from Sommerville, with 13 officers and 143 ratings either killed in action or massacred.

[8] The new fortifications were initially designed by Prussian engineer Hugo Strauss, later replaced by the Spanish Policarpo Fuentes,[21] and built in a mere twelve days by 800 workers.