In Normal mode, the player has to beat the zombies at Level 1.
There are now two steam Sewers on the board, which appear right where the zombies spawn. This could make it difficult for the player to see, but that's unlikely. Besides the Sewers, this level shouldn't be that hard.
In hard mode, the player has to defeat leveled-up zombies.
All zombies in this level are at level 4. This, when combined with the threat of Sewers, can make a pretty nasty scene on the board. Make sure you bring high-leveled plants before taking on this level.