Then, other zombies may use the ladder after it is placed, unless the plant gets removed or a Jalapeno, Cherry Bomb, or Cob Cannon destroys the ladder.
Plant two columns of Sunflowers while planting Potato Mines to kill the zombies at the start of the level. Then, start planting one column of Kernel-pults and one or two columns of Cabbage-pults. Plant extra Flower Pots when needed.
Then, plant two columns of Wall-nuts to defend yourself from dropping zombies at the final wave and other zombies coming from the right hand side of the board.
Use instant-kills to kill big groups of zombies or ladders when placed on your Wall-nuts.
If necessary, plant an extra Flower Pot and an instant-kill plant on it to kill extra zombies.