I am not sure if these guys are an official paid band or just buskers, but they were there all three times I was in the Prague Castle area. They are really good and sell their CD’s as well. I bet that if you visit the castle they were be there playing their tunes.
Update: here is a photo that my friend Hugh took of them in 2002, seems like they have been there for a while.
Shot on a 35mm film camera, with a 50mm lens, using C41 black and white film. Much as I love shooting digital, there is still a special place for shooting in film, and never knowing quite how it came out until a few days after shooting.
It really teaches you to take your time, frame your pictures, check your settings, and to get the shot right in-camera (and to make plenty of mistakes).