Between the 11th and 12th centuries, a slim tower was built on the south spur of the plateau in order to stand guard over the main routes of communication and to strengthen Cástulo’s defensive capabilities ahead of the advance of the Christian armies along the River Guadalquivir. It was built of rammed earth and mortar, with walls that grew thinner towards the top. The tower had a tiered internal layout and boasted four above-ground storeys and one below ground. Each storey was divided into two rooms by a central wall, which supported a connecting staircase. One of the rooms on the first storey contained a cistern, while the other was filled in to give the tower a solid base. The tower’s entrance was located on the second storey and accessed via an adjacent ramp.

After the area was conquered by the Christians in 1227, this slim Islamic tower was converted into the keep of a small castle, which consisted of a masonry wall protected by a series of square-shaped towers around the perimeter. The castle was separated from the rest of the settlement by a defensive moat, which was dug in order to reinforce the defences of the north side of the fortification. These defences were further strengthened by the addition of the stone towers. Materials discovered around the site show that the castle was occupied during the 13th and 14th centuries, after which it was abandoned for good.

In the 15th century Cástulo was used as a source of construction material, resulting in many of its structures being dismantled, and accelerating the castle’s deterioration and ruination. Although the large rammed-earth tower has been restored, only half of the structure is standing.

This site has been occupied since 3000 BC, when a settlement with an agrarian economy emerged and subsequently became linked to the mineral extraction activities taking place nearby, midway through the Bronze Age. By the end of this period it had become a notable population centre with a mixed economy based on the crop farming, livestock farming and mining activities carried out in the surrounding area. 

The mining operations in the Sierra Morena mountains continued into the Iberian period, making Cástulo one of the most important oppida in Andalusia. As the capital of the Oretania region, it minted its own coins. In 228 BC, the Carthaginians took the settlement on the orders of General Hamilcar Barca. 

In 206 BC, Cástulo was conquered by the Romans. The town exercised financial control over the metal deposits in the eastern Sierra Morena mountains, which brought it great economic and social prosperity, as demonstrated by the many public buildings that have been discovered through various archaeological excavations.
This prosperity continued into the Visigothic era and Cástulo was a bishopric until the end of the 6th century, when it began a long process of decline and was overtaken in importance by Beatia/Viatia (modern-day Baeza). The population also shrank in size, occupying only small areas within the old Roman town.

Cástulo remained occupied during the Islamic era, when it was known as Hisn Qastuluna, a small enclave that was essentially dedicated to crop and livestock farming but that still retained sections of its old defensive wall.

As the Umayyads consolidated their rule, they came into conflict with the tribal communities (fundamentally Berbers), who fought to remain independent. Between 785 and 786, the troops of Emir Abd al-Rahman I clashed with the Berbers commanded by Abu-l-Aswad, kicking off a period of struggle that ended with the defeat of the Berber forces at the Battle of Qastuluna. 

The town was also embroiled in the conflicts that took place between the 9th and 10th centuries, when Qastuluna was occupied by the Muladí rebel Ubay ben Alah ben Al-Shaliya. He went on to rule this area for approximately 20 years, until Abd al-Rahman III finally quashed his revolt and retook the town, after which Qastuluna was incorporated into the iqlim (district) of Bayyasa (Baeza).

Eventually, the instability of the Upper Guadalquivir region following the fall of the caliphate and the creation of the taifa kingdoms led to the fortification of many of the local hamlets. This process was accelerated by the raids and expeditions carried out in the area by the Christian armies, mainly during the Almohad period, which resulted in fortresses being constructed at strategic points along the main waterways of the Upper Guadalquivir region. The rammed-earth tower that would form the nucleus of Santa Eufemia Castle was built as part of this process of fortification.

After the Christians conquered this area in 1227, they mostly settled in the largest towns and fortified complexes; as a result, numerous rural settlements, hisns (refuges) used by the peasantry and other fortifications located far from the border were abandoned. However, in view of its strategic position overlooking the local routes of communication, Cástulo’s defences were strengthened with the construction of a castle that used the old Islamic tower as its keep. Despite this, over the course of the 14th century, owing to its distance from the new front line on the Baetic Mountains, Cástulo was definitively abandoned and used as a source of building material for the cities of Baeza and Linares. A century later, the order was given to demolish its walls and the remains of any other buildings that were still standing.