In brief
The Hellenistic foundation and medieval buildings belong to different layers.
Useful to know
UNESCO traces Trogir's orthogonal street plan to the Hellenistic period. The island settlement later received Romanesque churches and Renaissance and Baroque buildings from the Venetian period. [1]
The plan of the streets may be older than the facades you see along them. When considering, compare the network of directions with the dates of the individual monuments. This helps to understand the urban continuity without claiming that every stone is antique. The historic city retains relations between spaces, even if the buildings have been changed many times.
Sources
For further detail and to check the information: