Aínsa Castle is located at the western end of the main square in the Aragonese Pyrenean town of Aínsa, which is located in the Sobrarbe region on the banks of the Ara and Cinca rivers. It is made up of two very different stages of construction. The first is from the mid-11th century, when many points on the southern border of Sobrarbe were fortified to protect against Muslim domination. The second phase is the reform and extension of the 17th century, commissioned to the engineer Tiburcio Spanochi, and was carried out to fortify the border against possible invasions from the Pyrenees. During this period, the towers were built and the north wall was fitted out for artillery use. From the 18th century, the castle fell into disuse until it was used as a military fortress in the various wars of the 19th century.

The castle is the result of the combination of these two construction phases (mainly from the 11th and 17th centuries), which has given rise to a complex defensive enclosure. The different military and strategic needs of each period have significantly altered the construction. The engineer Tiburcio Spanochi, in the 16th century, in charge of the construction of fortifications in the Aragonese Pyrenees, conceived the castle of Aínsa as a unified enclosure, taking advantage of the remains of the previous stages and giving it homogeneity. The remains of the original medieval fortifications from the 11th century can still be seen today.

The current result is an enclosure with an irregular ground plan with towers at each vertex, built with ashlars and reinforcements at the corners. The wall, also built of ashlar, is imposing and reaches a height of 14 metres from the inside. The enclosure has two entrances, east and west, two bastions and a moat on the west side. The wall is accessible from the top and has many loopholes or loopholes for artillery along its length.