The Austro-Hungarian army approached the war as a scientific discipline. In the 1880s, when it was decided to strengthen the border with Montenegro, military engineer Karl Wahlberg designed a special type of mountain two-part carved stone fortress. Such was Petrina, which was originally built as a three-storey defensive barracks. After three years, two semicircular artillery platforms were added. This system of defense meant that the communication between the fortresses took place by optical connection, so that today at least two to five fortresses can be seen from each. On a tourist board at the foot of Petrina, the thematic story of Ivo Andrić begins, with the quotes from the book „The bridge on the Drina“, which describe the atmosphere of the arrival of Austria-Hungary in Bosnia and Herzegovina and the great social changes that followed.