This pattern holds anywhere we find a 1 next to a 2 against a flat wall of unknowns. Make sure the 4 spaces behind them are visible and can't be monsters!
First, this 1 tells us there is one monster in this oval.
So as there is only one monster on the left, the 2 tells us there is a second monster in this rightmost cell.
We also know the leftmost cell is safe from the 1 using the same logic in the other direction. We know there is a monster in the two spaces next to the 2.
