The Keep by F. Paul Wilson

This was a pretty good book I have been looking for a while. It is wartime Romania and the Nazis have taken possession of an ancient mountainside fortress covered in strangely configured crosses. When the Nazis are being picked off one by one, they send for the SS who in turn send for a Professor and his daughter who are Jewish to help them solve the mystery.....Pretty good read, a little plodding, but worth your while. You can order it from Amazon used for as little as a penny....