Europe's largest undestroyed fortress - the Petrovaradin Fortress in Novi Sad, Serbia - is bound to have a few stories to tell, and the GHI team want to hear them. Alen Panikovic, who works at the now-museum, shows the team the grounds, including a prison, a barricade area, an execution wall, and a tunnel system 16 kilometers long. The facility is so large that their normal equipment won't do the trick, but luckily Paul is able to rig a device for each of the investigators so that they can carry DVs, audio recorders, EMF detectors - the whole toolkit - on one hand-held rig. Genius. Barry warns them to be careful of the fortress's dangers, including collapsing tunnels, and with that, the explorers set off.