Literally and figuratively a right royal adventure, a tour of the catacombs beneath the Cathedral, available in several different languages, lasts for about 45 minutes and is well worth all the bending and ducking required to go on one. The tour guides are extremely well versed in history and hugely entertaining, and they even give out small prizes if you answer some of the questions they ask correctly. Tours begin at the ticket office inside the adjacent Vilnius Cathedral Bell Tower and should be booked in advance. See the website for exact days and times.