The act of fortifying consists of providing a building, urban nucleus or territory with defence works which can resist an attack by enemy forces. The site and form of the different fortifications has varied over the centuries, depending on geo-political balance, technology and the available resources.

The castros of the Second Iron Age (approx. fourth and third centuries BC) are the oldest fortified areas still extant in Gipuzkoa. Seven have been identified to date: Intxur (Albiztur), Buruntza (Andoain), Basagain (Anoeta), Murumendi (Beasain), Munoaundi (Azkoitia), Moru (Elgoibar) and Murugain (Arrasate).

16. Tower-house.

There are also many fortifications dating from the Middle Ages: dozens of tower-houses, town walls and castles in Hondarribia, San Sebastian, Mendikute (Albiztur), Beloaga (Oiartzun), Ausa-Gaztelu (Zaldibia), Jentilbaratza (Ataun), Aitzorrotz (Eskoriatza), Goikobalu (Arrasate) and Elosua (Bergara).

In general, mediaeval military architecture was characterised by stone walls (which were incombustible) 2 to 2.5 metres thick (sufficient to stop arrows and projectiles fired by catapults). They were very tall, and thus difficult to scale. At intervals towers rose above the level of the wall and abutted from it. Defenders stood atop a narrow adarve with a battlement parapet on its outer side. The gates were protected by a barbican (or fortified area surrounding the exterior of the entranceway, a drawbridge over a moat or fosse and machicolations (slightly projecting sections with a crenellated floor).

The walled towns of the Middle Ages not only served a military function; they also delimited legally separate areas. Tower-houses too formed part of the arrangement of the rural environment and reflected the social and economic prestige of their owners.