Start by naming any country that France borders. Then name any country that borders any of those countries. Then keep going until there are no more countries left to name.
I couldn't remember the Constantines, Donalds or Duncans and now I feel totally stupid. I don't think I would have ever got the Dubs and the Indulfs how ever long I thought about it though.
Really good quz. 'Cuilén' is notoriously hard to get, so maybe allow for 'Cuilen' or 'Cullen', and Duncan II who reigned briefly in 1094, should be included too.