Try the U Zlate Studny Hotel, which is just behind the Old Town Square and 5 mins walk from The Charles Bridge. If you "google" it you'll find their website, it's a beautiful 14th century building it has ordinary rooms, and one and two bedroom apartments. It's reasonably priced and the location is really brilliant, near all the life of the Old Town Square but behind it down a side street. It's within walking distance of both the Castle area (uphill) in one direction and Wenceslas Square in the opposite direction. The nearest Metro station is five minutes walk away. I was there with friends for the Christmas markets last year and it was perfect.