In Algeria, several old urban nuclei are decayed or severely damaged. Numerous are the factors related to these damages: the age of constructions (many centuries), the bad or absence of maintenance, the natural disaster (earthquake, landslide, and floods) and sometimes the abandonment. Since 2003, the law 98-03 related to the preservation of cultural heritage and its application decree was implemented. In fact the first safeguard sector plan in Algeria was applied to the Casbah of Algiers which is classified as a World Heritage. This old urban nucleus presents an important decay. The different problems related to its aspect are principally the age of constructions; some of them date from the 10th century and the majority of the 16th; the absence of maintenance and recently in 2003 the impact of the earthquake. The project is still under investigation and is established by the CNERU Group (National Center in Urban Research and Studies). Furthermore the work on these areas was done under constraints. The Casbah of Algiers is a World Cultural Heritage, what induced us the installation of emergency measures (propping up), then in parallel an in situ investigation in order to determine the vulnerability of constructions was done (minor and major inheritance, houses and buildings) in order to elaborate the manual of rehabilitation and the safeguard plan and its regulation. This paper presents the adopted methodologies in this area and the different actions as the emergency measures, the constructions propping up and the manual of rehabilitation.