Fortress of San Marko, Military fortress in San Sebastián, Spain

Fuerte de San Marcos is a stone fortress at 260 meters altitude between San Sebastián and Rentería, featuring a polygonal layout with multiple defense positions. The structure contains exhibition spaces, guard rooms, powder magazines, and an underground water cistern.

The fort was inaugurated in 1888 within the Campo Atrincherado de Oyarzun, a scheme meant to hold eight forts; only San Marcos, Txoritokieta and Nuestra Señora de Guadalupe were built. Its final project was signed after inauguration by engineer captain Luis Nieva, following proposals from 1878 onward by Pedro Lorente, Juan Roca, Francisco Echagüe and José Brandis. It was used as an arrest centre for non-commissioned officers and held by the army until the 1960s.

The fortress functions as a cultural center and hosts exhibitions about regional military history and defensive architecture. Visitors can observe how these structures shaped the landscape and defense strategies of the era.

The fort stands at 260 m on Mount Bizarain, on the boundary between San Sebastián and Errenteria. Access is via a tunnel linked to the bridge at the main gate; inside, a ramp-stair connects the upper and lower works. The surrounding covered path is 530 m long, and the moat is 8 m wide with walls 2 to 7 m high.

A 530-meter covered pathway leads to a tunnel connecting to the main entrance bridge, demonstrating advanced 19th-century military techniques. These underground connections reveal the sophisticated defense system that was engineered for the era.