Normal sudoku rules apply. In addition: Both marked main diagonals must contain the digits 1 to 9 once each; cells separated by a king's move (in chess) cannot contain the same digit; the grey dominoes form 2-digit prime numbers; and the cells with blue circles form a (slightly offset) magic 'square' (ie each row/col/diagonal must contain the digits 1 to 9 and sum to the same total).