In a cavern
A simple cavern used as a troops' hideout was usually driven from both sides, where cranked entrances were made. These were then connected by a larger corridor. However, unsuitable conditions were met during the construction sometimes, mostly represented by karstic features. In such a case has the work either stopped or constructors tried to include newfound subterranean spaces into the complex somehow. Many various and interresting underground objects have arisen in this way.