Three kilometers as the crow flies from the Palazzo di Baldu is the Castle of Balaiana, connected by a path to the Church of San Leonardo, which was originally the castle's chapel.
The structure is located on Monte San Leonardo at an altitude of 299 m above sea level and dominates a large part of the territory and the roads that connect the ports to the towns of the Judicate. For this reason, the castle served to defend the internal borders and the surrounding area.
The fortress was probably built by Pisan craftsmen in the course of the 11th century on the orders of Giudice Constantin I. This hypothesis is also supported by some structural features; the castle was built entirely of hewn granite blocks and without the use of mortar. The castle is characterized by a trapezoidal tower divided into two rooms, with access on the east side.
The presence of aligned stones in the surrounding plain has led to the assumption that there was a ring wall to prevent advances from all directions. The Castle of Balaiana is mentioned in the Condaghe of Bonarcado (first decades of the 12th century and mid-13th century), which reports the clashes between the Giudice of Gallura, Constantine III, and the sons of his predecessor, who occupied the fortress had. It was then given to the monastery of Santa Maria di Bonarcado in 1146 by the barisone of Arborea; their destruction is carried out by the Catalans/Aragonese. Due to the poor state of preservation of the castle, extensive restoration work was carried out at the end of the last century.