This line consisted of five fortified elements aligned in a SE-NW direction: an armoured redoubt blocking the Endarlaza bridge, a fort on Mount Pagogaña and three towers, all designed between 1878 and 1879. In the opinion of the French military intelligence, these fortifications might be considered "...of almost no value in themselves and insufficient against any troops provided with artillery...". Their only function, it was felt, might be to hinder Carlist movements in the event of another war.

124. Endarlaza Line:
1-Endarlaza Gateway (no remains);
2-Endarlaza Tower;
3-Pika Tower;
4-Pagogaña Fort;
5-Erlaitz Tower (no remains);
6-Excavations and barrack huts of the Erlaitz Fort;
7-GI-3454 road;
8-N-101 road;
9-Border;
10-Bidasoa River.© Juan Antonio Sáez

The armoured redoubt of Endarlaza was rectangular in shape, 7 metres by 2 metres at the base and 3 metres in height. It was reinforced with sheet metal and crenellated. Raised on stone pillars, it allowed traffic crossing the bridge to pass underneath. It was considered to serve no military purpose and was demolished in 1903.

125. Pagogaña Fort (in 1916):
1-Troop barracks;
2-Communication between the four adjoining bays;
3 Kitchen;
4 Effects;
5 Sergeants' quarters;
6 Washroom;
7 Rifle gallery;
8 Corps de garde;
9 Provisions store;
10 Officers' quarters;
11 Commander's quarters;
12 Officers' mess;
13 Courtyard;
14 Stairs between the two floors in the central tower.© Juan Antonio Sáez

The towers of Endarlaza, Pika and Erlaitz were located at a height of 30, 224 and 500 m respectively at the watershed of the Endara basin. The three were round in shape with an external circumference of 26 metres and ordinary masonry walls, 60 cm thick. They had two storeys and a flat roof. Entrance was by means of a retractable metal staircase leading to a door on the upper floor. The lower floors had 17 embrasures, over four of which there were skylights. The upper floor, with four square windows and the main door, had fewer embrasures.

Nothing now remains of the Erlaitz tower, which was demolished in 1891 when work began on the fort of the same name. There is documentary evidence that it had a fosse and that it housed the line's semaphore station. In 1915 the remaining towers were already abandoned and in poor condition. The enclosing walls are still standing.

126. Armoured redoubt of Endarlaza (from French report):
A Elevation:
1 Metal roof covered in tarcloth;
2 Walls made of an oak structure covered in riveted sheets of metal;
3 Reinforced door with a loophole;
4 Retractable metal stairway leading to the door;
5 Metal gate capable of blocking traffic across the bridge;
6 Stone pillars supporting the redoubt;
7 Crenellations (7 on the longer side and 2 on the shorter ones). B Plan:
8 Longitudunal supporting beams;
9 Fourteen embrasures in the metal floor;
10 Corbels supporting the crossbeams of the base of the redoubt.© Juan Antonio Sáez

The Pagogaña fort is the most important fortification on the line. It consists of an octagonal tower of ordinary masonry with groins (corners made out of stronger materials than the rest of the construction). It has two floors and a crenellated roof. Four rectangular single-storey bays extend from each of the four longest sides, with two-sloped roofs, making the entire complex cross-shaped with a capacity to house a garrison of 84. A small shooting gallery linked two arms of the cross, providing four embrasures.

127. Pika Tower (Irun). The tower was originally covered in plaster, concealing the ordinary masonry and brick.© Juan Antonio Sáez

The fort had a glacis and a triangular-sectioned fosse, 3.75 m wide at the top. Rainwater was gathered in a cistern to be used by the garrison.

In May 1882 infantry were stationed in the fort and six years later part of the fort was handed over to the Carabineros, who finally took over the entire complex in 1893. In 1916 it was already in poor conservation. It was badly damaged during the Spanish Civil War (1936-1939) and only a few remains can now be seen.