ENThe article analyses work journals of a great researcher and architect Sigitas Benjaminas Lasavickas focusing on the preservation projects of the ruined defensive walls of the Medininkai Castle. The first major research and conservation works of the Medininkai Castle including preservation works of defense walls that started in 1954 and 1958-1961 were conducted by Lasavickas. A lot of the research findings and processes of these works were only documented by Lasavickas, therefore, his work journals is often the only source of information uncovering many long forgotten facts. Lasavickas journals unveil the process of the studies conducted at that time as well as why certain conservation methods and work processes were chosen. Lasavickas was one of the first researchers who thoroughly analysed the architecture, building materials and technology of the Medininkai Castle. Based on the analysis of Lasavickas journals the article shows how much Lasavickas ideas influenced the decision to proceed with the conservation works (e.g. fragmented restoration of masonry just to prevent further collapses and leaving uncovered authentic fragments for visitors) rather than reconstruction of the Castle. The article then describes the evolution of the conservation approaches and techniques and, finally, concludes that new approaches to the conservation work that have developed in the last decade of XX century would not have been possible without the findings and works done by Lasavickas in earlier decades.