Urban Planning / Urban Design
Keep in mind that almost any older city is a blend of different cultures. Tunis was Phoenician, Carthaginian, Roman, Traditional Arab-Islamic, and Modern, and there are elements of all of those infused. Istanbul is another great example.
Well as an urban planner in the Gulf, we had to do just that. Our goal as part of Abu Dhabi 2030 [skip to about 2:00] was to create an Arabic city of modern cultural heritage. This included blending the modern structures of the West and Asia with traditional heritage design of the Bedouin.
The answer to your boundary lines is that there likely won't be physical boundaries. You start to see a mix of the two cultural architectural traditions throughout the urban fabric. For example, a metro station that uses mashrabiya design from the tenth century for passive shading in the desert heat. Elsewhere along the beaches we instilled the designs of traditional dhow (fishing boats) sails into the covers of buildings such as the Formula One Race track.
The latter is very much 'modern meets tradition,' but this can also happen with two parallel cultures. The gulf culture prefers high separation to public and private space. So tall modern complexes had unique blends of design alterations to continue this blend of west meets middle-east.
To learn more about this particular combination of cultural design smash-up (Western vernacular with Arabic colloquial), you can visit the projects we did. Otherwise, any Urban Planning book (my favorite is Cities of Tomorrow) will give you a historical perspective of how the architectural and spatial vernacular of cities were arranged.