The octagonal tower of the ruined castle Ojców (southern Poland) is considered one of the most impressive foundations of king Kasimir III the Great (†1370). The purpose of this paper is to discuss the results of archaeological dig carried out in 2016 and to combine it with written evidence to form the basis for the reconstruction of its long-gone past. The tower is an octagon in plan, has 11.55 m in diameter and has walls that are ca. 2.7-2.8 m thick. On the inside it is round, 6.2 m in diameter. It was built of carefully processed hewn limestone set in lime mortar and what is important to note, it is the only known castle tower realisation of king Kasimir which was built on such blocks. On the other hand, the raw material used for building the tower has close affinities with church foundations of the king (e.g. in the collegiate church in Wiślica). Noteworthy, the fieldwork of 2016 provided rich assemblage of architectural details, including stylistically homogenous window or portal framings with characteristic pear-shaped mouldings and hollow-chamferred profiles, which likely relate to the earliest stages of the castle, perhaps already to the realisation of king Kasimir. There is a high degree of confidence that these elements were originally placed in the tower, and, if so, they determined rich and representative design of the whole structure. According to the author of the paper, there are strong indications that the impressive octagonal tower which is distinguished among the other contemporary defensive realisations by its building material, size and, possibly, a decor, was built as a commemorative realisation, given to honour the memory of the father of king Kasimir – Władysław the Elbow-high, who according to the local tradition, early in the 14th c. found a refuge in a cave located nearby (note the castle’s name: Oczecz − further Ojców − in Polish means Father). Last but not least, the archaeological dig brought to light the remains of an undefined building from the late 15th-mid 16th c. west from the tower, the remains of post-medieval (17th c.?) wall adjacent to the tower from the north, and some important stratigraphic observations, which allow to state that the octagonal structure witnessed some extensive restoration work in the second half of the 15th c.