Normal sudoku rules apply. Cells in the same relative position in different 3x3 boxes cannot contain the same digits. (So, for example, the top left cells in each 3x3 box must all be different.) Digits on a circle have to be placed in the same circular order in the four cells that are touched by the circle. The circle has to be rotated 90° clockwise N times from the current position. N is the digit in the cell inside the circle. Eg, if R2C8=6, then R1C8=8 and R2C9=4.