When you look at the map, and you don't see a white border in a specific room (left, right, up, or down), it means there is a part of that room you haven't explored yet, and you have to head in the direction where the white border is missing. Usually this helps to acces completely new parts of the castle. This way you can also tell where most likely there is something blocking your path, and for which you need a special item or ability, though it's also possible you might have just missed an exit or something. By means of elimination you can progress further and further.