Abstract The background for the establishmentof prison Class IIA in Purwokerto has been encouraged by three things. Firstly, the excess of the capacity of the prisoners which has already increased into 300%.  Secondly, the  lack of the facility for assimilation  activities . Thirdly, the aspect of security which is considered as the most crucial aspect of the establishment of the  prison . As considering those three backgrounds, an adequate facility that can give guarantee  to the safety of the prisoner as well as their  activities and indirectly can accommodate the assimilation activities shall be created. Moreover, the Emphasis on security systems in the establishment of the prison shall be concered and well implemented, as security aspect is the most important aspect in the architectural plan of the building. Furthermore, the Security aspect is divided into internal security, external security and security of the building. The result is the design of a prison that embodies four main necessities, such as public facilities, detention facilities,closed  prison facilities and Open Prison Facilities. Finally, the arrangement of the capacity of the prisoners for short term and long term, the availability of the sufficient facilities that can accommodate the activities in prison, and the security aspect of the prison are put in  the design planning of the building, so that an idealand safe prison that can facilitate as well as accomodate the assimilation activities of the prisoners can be attained.  Keywords Architecture, Prison, Overcapacity, Assimilation, Safety System
                        
                        
                        
                        
                            
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