A city appears as a palimpsest of different ages through its built environment, which is the visible indicator of the passage of time and the medium of collective memory. The ruins form the lower layer of the palimpsest, being extracts from the past, and are presented as inactive spaces- heretopies of the public sphere. Namely, they represent elements that are encountered with special treatment and are illusory cracks of the traditional time, because of the ruination and the associations that are produced. Within this framework, in the first chapter we search for the characteristics of the natural /artificial ruin with reference to the definition of both of them, the ways they have been used, and we consider the stages of the natural / artificial ruination. In the second chapter, we analyze the role of architectural ruins in the 17th and 18th century, a period that produced new thoughts on the ruins. We study the relationship between the ruin and the collective memory, the aesthetic th