Logic Masters Deutschland e.V.

Constantinople, by James Sinclair

Normal sudoku rules apply within the 9x9 grid.
Skyscrapers: digits outside the grid indicate the number of digits in the 9x9 grid that are visible in that direction, where higher digits block the view of lower digits.
Counting Circles: a digit in a circle (including an arrow circle) indicates how many circles in the puzzle contain that digit.
Arrows: the sum of the digits along an arrow is equal to the digit in the connected circle, and these digits can repeat if allowed by other rules.
Digits in cells separated by an X sum to 10.
Digits in cells with a shaded square are even.
Digits in cells with a shaded circle are odd (these clues are not included as counting circles).

SudokuPad

For more of my puzzles, check out Artisanal Sudoku, where I publish a new set of (generally pretty approachable) variant sudokus every week.