You are the river. Each day, before the raiders march, use your moves: open a gate, cut a bank, call the rain, freeze a reach.
Water runs to lower or level ground and never uphill. It drowns any raider it reaches, and ruins any cottage too.
The red lines show exactly where every raider will walk, and which cottage it will burn. Change the ground and the lines change.
Hold out to the last day with a cottage standing and the month is kept. Undo is free until you end the day.