In a multicultural world, the critical view of modern Arabian cultural facts reveals a great dilemma. The everlasting problem is the analogy (mimesis) of the west culture and the coincidence with its vision or thoughts.
Dangerous and contradictory dualities had appeared in the deepest Arabian modern culture such as authenticity-modernity, self-other, past-present; state that had been reversed on the city. The traditional city had suffered from this situation and was an essential motivation to formulate this paper, adding the absence of a global theoretical framework for the definition of discontinuity. Thus, the coincidence between self and other produced a discontinuity or a rupture because of the breakthrough of intellectual concepts which had changed the existing urban fabric.
The aim of the paper was to detect and to define the most important catastrophic event in the old traditional city as a physical discontinuity in urban design.
The paper uses two computer’s aided programs as tools to resolve the problem: 
•	to design of a program on Delphi 6 based on the Space Syntax Theory – of Bill Hillier- to calculate the global and local spatial qualities ,
•	then calculating the critical set of values by studying the behavior of the urban system through the Equilibrium System using Catastrophe Theory (Cusp Catastrophe) – of Rene Thom- aided by the program of Matlab 5.3 .
The elected sample was selected in Casbah of Algiers and defined for the application study.
The research‘s results revealed that the ingression of modern fabric and its diffusion in (1880) leads to the most important and greatest catastrophic event in the traditional city in terms of nature and size. It also appears that another important catastrophic event was revealed in (1830) which showed that some axis in the old fabric were in disequilibrium state and has caused a catastrophic event in (1830).
