The first three rules in the catalogue work against you. The bell stone is on your side: it is the first tool that lets you change the board.
Behaviour
You throw a stone you found in the level at any node at most three edges away from your own. Throwing takes the place of a move.
Every sentry, patrol and chaser within two edges of where the stone lands enters a "distracted" state for three turns: it abandons its route or its post, walks the shortest path to the node the stone landed on, and when the time is up returns to its duty the same way.
A distracted entity's sight is only the node it stands on — its chain does not run, because it is looking at a sound.
Two finer rules: distraction overrides a chaser's alarm; and a patrol returning from distraction does not make up the turns it missed — it rejoins its route at the nearest stop.
How you read it
When you enter throwing mode the reachable nodes are highlighted, so you never have to count three steps yourself. Where the stone lands, the area of effect is shown with a ripple, and the distraction timer counts down from three above each entity.
Taught in
Taught in2-6
This is the strongest tutorial level in the catalogue: take the rule out of the data and the level becomes unsolvable. The sentry on the way can only be moved by sound. Which makes the stone not an option but a key.