The mediaeval fortifications of Hondarribia (Fuenterrabia) were enlarged as the population grew and the town's strategic importance increased. The Catholic Monarchs undertook significant improvement and enlargement, although these constructions had to be repaired at several points due to the ravages of war. Charles V once again renovated the fortifications and in the second half of the sixteenth century engineers El Fratín and Tiburcio Spanocchi carried out additional work.
Hondarribia was surrounded by a modern, roughly pentagonal, fortification, dominated to the west by hills and surrounded on all other sides by the sea. Clockwise, the bastions were: St. Philip's (of which some remains can still be seen), The Queen's and Leiva, the Mary Magdalene Turret (no remains now visible), the Old Ammunition Tower (no remains), St. James' Bastion (semi-ruined) and the small Bamba Tower (demolished in 1729). Thick walls run from St. Philip's Bastion to the Mary Magdalene Turret, criss-crossed inside by sections of tunnels and countermines (internal vaulted passageways whose function was to prevent the besiegers from digging underground tunnels (mines) to enter the enclosure or place explosive charges beneath the walls).
The section of wall between the Old Ammunition Tower and the Mary Magdalene Turret was the weakest point in the fortification, since on this side the sea provided natural protection.
The walled city had two main gates, St. Mary's and St. Nicholas'. The former, which stood close to St. Philip's Bastion, led into the town along the main street (Calle Mayor). The second was located between the Queen's Bastion and the Leiva Bastion. A ravelin was erected in front of St. Nicholas' Gate and another (the Guevara or Mary Magdalene Ravelin) between the Leiva Bastion and the Mary Magdalene Turret. These were practically demolished during the French siege of 1719.
The western inland section was crossed by a fosse (which also surrounded the ravelins), a covered way (with several parade squares and traverses) and the glacis facing outwards.