In Fences it's currently not possible to place crosses as a reminder that there cannot be a fence in that position. The placing of crosses can help solving the puzzle. Is that a possible feature?
btw I was looking for slitherlink which is a different name for fences.
Hello, jeoen! As the author of four of the Fences puzzles on this site (as well as a bunch that are too large to publish on PuzzlePicnic!), I sincerely hope you enjoy these puzzles and this site!
Also, what Johan said about the right mouse button.