Originally, all the fortifications, except maybe some small towers belonging to some familiar groups in the early years of the Reconquest, were royal dominions and the king appointed several noblemen as "tenentes" (the one who has the fortress), but this title was not hereditary. Castles continued being considered as the crown's possessions even when these buildings were transferred to noble families. So, the king maintained their right to enter and to stay in any fortress of the kingdom. The castles' governors had the obligation to give free entry to the king regardless the homage paid to their feudal lords.

Theoretically, any castle or fortress erected in the kingdom should have royal permission, otherwise castles were under threat of demolition. Successive agreements of the Cortes (parliament) of the kingdom had forbidden the construction of fortifications without royal permission, regardless they were castles, peñas francas, caves or knolls, and the person who "inhabited a king's old castle or peña brava without permission" was even considered a traitor.

The techniques of construction used were diverse and depended on the technical advances of that time, and on the availabilility of labour and building material of the site where the fortification was to be erected. The "tapias", walls built by means of plank linings either with mud , with or without stones, or with lime and pebbles, were the most common construction system used in medieval walls. In many fortifications of mudejar tradition, these walls had layers of brick and buttresses of the same material in the corners. Works made out of adobe were also profusely used in additional defensive walls in front of the gates against artillery between the late 15th century and the early 16th century.

The stone masonry, however well-shaped the stones were, contained a core of rubble mixed with lime, which constituted the main part of walls; in many cases the durabilility of the stone masonry depended on its right joining with the outer sides of stones.

The funds to build or repair the walls and castles of towns used to come from special taxes that were proportionately distributed among the inhabitants of towns and small villages dependent on its alfoz ("repartimientos"). In some occasions, a section of the enclosure was assigned to each collación (parish) or village rather than sharing out the total costs. Neither clergymen, nor jewish, nor noblemen resident in the town were exempt from that payment.

The sisa also used to be applied; it was a temporary purchase tax on essential goods, such as fish or meet. The amount of the caloñas(obtained from fines) was sometimes allocated for the conservation of walls.