Normal sudoku rules apply. Minimal between lines: Digits on a line must be between the digits in the connected circles. The difference between the circled digits must be as small as allowed by normal sudoku rules. For example, a circle in R1C2 connected with a straight line to a circle in R5C6 would have a difference of 2 because it is possible (and necessary) to repeat the same digit in all 3 spots along the line. Clones: The grey 4x4 regions must contain the same digits in the same arrangement. Skyscrapers: Each digit N in the grid represents a building of height N. Numbers outside the grid indicate how many buildings can be seen from that direction in the row, with taller buildings blocking shorter buildings from being seen.