Normal sudoku rules apply. Divide the grid into cages, i.e. non-overlapping areas of one or more orthogonally connected cell(s) with no repeating digits. A clue in the top-left corner of a cell indicates the sum of digits in that cage. Some cages do not contain any clues. For these other cages, the sum must be determined by the solver. Some cells may be outside cages. No two cages can share the same sum. Each cage represents a building whose height equals the sum of its digits. Clues outside the grid indicate the number of buildings “seen” from that direction. Taller buildings block the view of smaller buildings. Cells outside cages cannot be seen.