The Basilica of Saint Michael is an early Palaeo-Christian church which is believed to date to the 5th or 6th century. A mosaic unearthed in the basilica also demonstrates how ingrained Christian culture later was with the early Byzantine Empire.References:
Considered to be one of the most imposing Roman ruins, Diocletian’s palace is certainly the main attraction of the city of Split. The ruins of palace, built between the late 3rd and the early 4th centuries A.D., can be found throughout the city. Today the remains of the palace are part of the historic core of Split, which in 1979 was listed by UNESCO as a World Heritage Site.
While it is referred to as a 'palace' because of its intended use as the retirement residence of Diocletian, the term can be misleading as the structure is massive and more resembles a large fortress: about half of it was for Diocletian's personal use, and the rest housed the military garrison.
The palace has a form of an irregular rectangle with numerous towers on the western, northern, and eastern facades.