Some say that an Englishman's home is his castle. Logically, Czechoslovakia's two successor states, the Czech and Slovak Republics are both likely to be candidates for the title of most densely castellated country in Europe. Let them squabble over that. Inevitably, one will win the title. Personally, I reckon it will be the Czech Republic, because Slovakia's mountainous regions make unsuitable terrain for conquest or castle-construction.
Density can be either uniform or patchy.
Uniform and homogeneous density is like the density of raisins in a well mixed fruitcake - the same in any slice of the cake. With a separate history for 700 out of 770 years, the Czechoslovak fruitcake was not mixed well and long enough. The density of castles in a landscape stems from geography and from years of squabbling over land. In Central Europe, squabbling is a national pastime. Just as each landowner in fairytales was master of all he surveyed, each castle was dominant only as far as the eye could see.