Information on visiting real-life fortresses in the Czech Republic?

As he walked out into the twilight, the fog seemed to embrace him, no longer protecting a secret, but letting a story finally be told.

The air inside was stale, trapped for nearly a century. His light revealed narrow, concrete corridors. Stará Hůrka was built to hold, to fight, and to protect. In the main artillery chamber, he found what the diary had hinted at. It wasn't gold or weapons. It was a perfectly preserved, airtight metal box.