The volume includes the abstracts and the lectures in their original language that were presented at the 15^th edition of the Conference Series on Theoretical and Practical Issues on Built Heritage Conservation – TUSNAD 2011, which was held in Alba Iulia and Şimleu Silvaniei, on the topic Fortresses once again in use. The lectures and the discussions were centred around the complex problem of the protection of fortresses, from the point of view of specialists of different areas, including a biology and building physics.