In 1898 war was declared between Spain and the United States. Coastal defences were hurriedly reinforced to keep the American navy away from the shoreline and prevent any possible landing.

188. Battery of the Provincial Government in Monpas. Emplacement on the left-flank with 15 cm 'Ordóñez' tubed iron cannon on a high carriage.

Once this initial work was practically complete, a "Project for improvement and increase of the maritime defences (of the Gipuzkoan coast) using locally available artillery", was drawn up. The project was approved in March 1899, when the war was already over. The plan envisaged reform of the Queen's, Prínce's and Bardocas batteries on Mount Urgull in San Sebastian and the building of the Mompás battery in Ulía. The battery in Astillero was to defend the port of Pasaia while the harbour in Zarautz was to be defended by a battery at the western end of the beach.

The work was to be financed by a donation from the Provincial Government of Gipuzkoa (300,000 pesetas) and by various town councils, under a subscription scheme begun to finance war expenses.

189. Battery of the Provincial Government:
1-Left flank emplacement;
2-Terreplein;
3-Vanguard emplacement;
4-Ammunition dump;
5-Right flank emplacement;
6-Powder magazine;
7-Postern gate;
8-Rearguard emplacement;
9-Ammunition dump;
10-Postern gate at the rearguard emplacement;
11-Access path 12-Terreplein;
13-Annex quarters;
14-Barracks;
15-Vestibule;
16-Excavation for an unfinished construction;
17-Control hut;
18-Path to the rangefinder station;
19-Rangefinder station;
20-Containing wall.© Juan Antonio Sáez

The most significant construction was the Mompás battery (renamed the Diputación [Provincial Government] de Gipuzkoa Battery in 1909). It consists of four pits: one in the vanguard, two on the flanks and one in the rearguard, with access by means of two posterns. The artillery consisted of four 15-cm tubed iron cannons (Ordóñez) with a range of 9 kilometres.

The complex also had a small barracks with accommodation for a garrison of twenty soldiers (replaced in 1909 by one with a capacity of 60), a command post (at an altitude of 64 metres above sea level, opened in 1917), a rangefinder station (alt. 73 m, opened in 1914) and a reflector, installed on Mount Urgull.

The Astillero Battery was sited on a small hill overlooking the canal leading to the port of Pasaia. It was intended to act as back-up to St. Mark's Fort and the auxiliary battery of Txoritokieta in defending the port. It consisted of a central traverse, under which stood the ammunition dump, and two platforms on either side on which four 15-centimetre hooped iron cannon could be mounted on a low gun carriage. The speedy conclusion of the war meant that these guns were never actually put in place. No remains can now be seen of this structure.