I was in Przemysl last year and a relative took me around showing me all the old forts and bunkers from WWI and WWII. This is my favorite because it is the biggest. Its very tucked away so you might want to do some research before hand how to get there. There is no entrance fee or anything since it is abandoned and overgrown. I suggest taking bug spray because the mosquitoes were quite bad. The fort itself is awesome, a labyrinth of tunnels. There are 3 floors above ground to explore, there used to be 2 floors below ground but they hae been filled in and are inaccessible. Its almost like something from a movie with ruins all overgrown in the middle of the forest.