i read about your turk problem , i think the game already has them as a civ , only they are called Ottomans .
dunno if they can be scripted to target certain cities , but having them go after Cairo , Constantinopole , Belgrade , Budapest and Vienna would make them amazingly historical.
also , perhaps using an imobile unit to guard Constantinopole against all units but cannon.
Dale has a Maginot line unit which you could adapt.
hmm, i found it distressing that almost all civ maps are wide open and any city can be attacked from 6-9 directions , and if you try to setup forts as a frontier , the AI would just move the units out. maybe use more mountains and lakes ?