Note: This is only when played for the first time.
The small number next to the zombie icon means that zombie will always appear in that lane, with 1 being the topmost lane and 5 being the bottommost one.
This level is not impossible to lose as the player can leave their defenses wide open and the zombies could walk through them.
Strategies[]
Placing Sunflowers in the first two columns closest to your house and then one or two columns of Peashooters should be enough to beat this level.
An alternative strategy is to plant three columns of Sunflowers, and then planting Peashooters in the front. Three columns of Peashooters can be done before the level ends.