Diocletian's Palace in Split was built at the turn of the 4th century AD as a residence of the retired Roman emperor and the founder of tetrarchy Diocletian. Due to its preservation, the amount of conducted research and published material, it is an excellent point of reference in the research of typologically and functionally similar structures. This Roman palace developed into a mediaeval municipality, and later into the modern-day city of Split. The continuity of life in the Palace has caused a lot of damage to the original layer, and each new intervention to the old city core leads to new findings. The systematization of findings offers an insight into the bigger picture of the former imperial palace and its interior design, which was used for both political and religious purposes. This paper contains the cataloguing and the systematization of the material according to the existing groups of decor: architectural relief, sculpture, fresco painting, mosaic and marble revetments. In addition to the existing historical insights and previously established facts, it contains the findings obtained during the last decade. These findings are the result of both archeological research, as well as intensive construction and conservation-restoration work within the old city core. New data has been obtained by analysing documents compiled by institutions which have been actively studying, documenting and working towards the protection of Diocletian's Palace: Split Department for Conservation, the City of Split's Department for the Old City Core and Croatian Conservation Institute. At the same time, museum materials have been revised. For the purposes of this paper, a field research was also conducted: stone sculptures built into Cambi Palace, which are believed to be a part of the original Diocletian's Palace decor, were taken down and restored. Using restoration procedures, the goal was to put a stop to the advanced stone degradation, as well as to analyse the form, period features and the type of materials. The findings of the whole research are heterogeneous and include both the interpretations of theories, as well as natural-science analyses of stone samples. In addition to finding new artifacts, new conclusions have been presented regarding stone-cutting workshops which were active in the Palace. Based on archeological findings and natural-science analyses, it can be assumed that the so-called "Brač workshop", with its distinctive style, was also active in decorating the Palace. By analysing each type of decor, it is possible to distinguish different phases in the development of the Palace. The overall revision of the material has led to a conclusion that there was a certain casual manner in creating the decor, as well as a decline in the quality of its execution. The transcendental trend which was present in visual arts had an effect on both iconographic canons, as well as previously strict technical standards. Sculptures and reliefs are unfinished, in places only roughly outlined. Figural motifs appear without strictly prescribed parameters, while certain important iconographic motifs were multiplied (thus losing their significance) and appear peripherally. A large number of marble revetments was cut roughly and remained without a finish. Wall and floor mosaics were made using large squares, and in some of the areas they were executed in a rough manner. The decadence of style should not be attributed only to speedy construction. The incompleteness and the apparent carelessness were an attempt to dematerialize the form typical of tetrarchy and its symbolic views. Taking into consideration all of the above, spacial illogicalities and the delevelling of the whole Palace should be interpreted as well.