Saint Mark’s Fort was constructed during the Second Carlist War to support the claimant’s mobile troops. It became a strategic point due to its proximity to Renteria, the Port of Pasajes, the railroad, and the main road.

Although at first it was known for its simple construction, basically made only of trenches, after the war it was rebuilt into the current fort. The construction was entrusted to Captain Luis Nieva and was part of the process of military reorganization of the area between San Sebastian and Irun that took the name of “The Trenched Field of Oyarzun.” The project had a budget of over two million pesetas and was completed in four years.

The fort is divided into what Luis Nieva called the upper construction and the lower construction:

Noteworthy in the lower construction is the barbette gun deck facing Oyarzun.

In the upper construction we find the top floor, the lower floor, and the patio:

o Top: Formed by 15 casemates with their corresponding embrasures and pieces of artillery. These casemates are covered with cement half-barrel or conical vaults and have windows in the wall that give onto the patio. Taking advantage of the thickness of the pillars some hollows are cut through that function as provisions, two of which had hoists that directly communicated with the supplies warehouse on the lower floor.

o Lower: Noteworthy are the Guard Corps and the Pavilions of the Governor and other officials.

o Patio: It is the most noteworthy part of the fortification, under which is found a reservoir made up of two tanks with their respective pumps and a capacity of 100m3. This allowed a garrison of 200 men survive without rain for 143 days, almost 5 months.

o Moat: The Fort is surrounded by an eight metre wide moat, except in front of the caponier and half caponier, where the width is reduced to six meters.