Place castles so every numbered sentinel sees exactly the number of spaces on its clue.
A sentinel sees its own cell and every non-castle cell in the four orthogonal directions. A castle blocks that direction and is not counted.
Castles cannot share an edge, but they may touch diagonally.
All non-castle cells, including sentinel cells, must form one orthogonally connected area.
Mark a cell as open space when it cannot be a castle. A clue turns green when its confirmed open sightlines reach its target and red when the target can no longer be met.
A low clue needs nearby castles to stop the sentinel’s view; a high clue needs long open sightlines. Begin where there are few possible ways to reach the number.
Example: A corner sentinel that needs 3 already sees itself plus two edge directions. Work out exactly where each direction must stop before considering a more open clue.
A castle can stop sightlines for every sentinel in its row and column. Check all of those clues before placing one.
Example: If a proposed castle shortens one sentinel to its target but makes a second sentinel in the same column see too few spaces, that castle cannot go there.
Open-space marks show the portion of a sentinel’s view that is certain. A green clue has enough confirmed open cells; red means the current castles have made its target impossible.
Example: When a clue turns green after marking a run of open cells, any remaining unknown cells on that sightline still need to be resolved without reducing that confirmed count.
Castles may touch diagonally, but never share an edge. A placed castle rules out its four orthogonal neighbors.
Example: After placing a 🏰, mark the cell above, below, left, and right as open space when they are editable.
All non-castle cells must form one connected area. Do not use castles to divide the board into separate islands.
Example: Before confirming a castle, check that every open area can still reach the rest of the board by moving up, down, left, or right.
A cell lies on one row and one column, so its state often affects several sentinels at once. Use one clue’s forced castle or open space to narrow the others.
Example: A castle forced by a horizontal clue may end a vertical sightline too, immediately deciding which cells beyond it must stay open.