The basics. It was one of many fortifications built by the Czechs in the time between WWI and WWII, but one of only five of this type of massive artillery fortification. Built in less than two years 1936-38, it could hold a garrison of over 300 men (hot bunking) who could survive sealed off for a couple weeks if necessary (they had a method of basically collapsing the entrance on itself). The main tunnel corridor pictured below is about 1/2 a mile long and has over 180 feet of solid rock above much of it. Although the structure was completed before the Czechs were sold out to the Germans by Chamberlain, the massive retractable artillery gun which the bunker was designed to control was never completed and installed. You can read more facts and figures and historical context on the linked page. If you click around the Czech language portions of the web site, you can see more photos and diagrams.
Here are a couple of photos I took during our tour.