One of the great things about Plants vs. Zombies is that it introduces new plants with each succeeding level. So you'll start with the Peapods, and the next level introduces the Potato Mine, which is cheaper than the Peapod, but takes a little while to arm itself. Once armed, any zombie walking into it is obliterated with a pleasing SPUDOW! The Potato Mine is a one-shot, so it's important to place it strategically.

It should be mentioned here that the horizontal zones are further subdivided into nine discrete squares. The zombies move steadily and linearly across these squares, but you can only add plants into one of the squares, never between. So you can only have nine plants in a row at most. At least one of these squares per row will contain a Sunflower; there's just no other way to afford your legions. That leaves eight squares to fill with other plants.

Other plants include the Snow Pea, which shoots icy peas that damage and slow down the zombies; the Cherry Bomb, which can take out a few zombies at a time; the Wall-nut, which acts as a defensive wall through which the zombies must chew; the Chomper, which can eat a whole zombie, but takes a while to chew it up, during which time it's useless against other zombies, and many others -- up to 42 different plant types in all.

Unfortunately, there are also a total of 26 zombie varieties to face off against. Zombies get tougher to kill when they're wearing items on their heads like a traffic cone, or a metal bucket, and later waves hold items in front of them like a screen door or a newspaper. There are plants that can attack a zombie through the screen door, and while the newspaper doesn't really provide that much more defense to the zombie, once it's gone, the zombie moves much faster than the other zombies do.

Each new level brings new challenges and new ways to manage them. There's a pole-vaulting zombie that can jump right over a Wall-nut or a Potato Mine, but otherwise is just as vulnerable as any other. Later levels take place at night, meaning that there's no sunshine dropping from the sky; all sunshine must come from Sunflowers. However, you do get access to small yellow mushrooms that provide lesser amounts of sunshine at a lesser planting cost, and tiny purple mushrooms that don't cost anything and fire weaker projectiles with less range than the average shooting plant does.

As your plant arsenal grows, you'll run into the added necessity of having to choose which plants you want to deploy; you can only have six available at a time, though later levels raise this amount. You'll also be given a shovel with which to remove a plant from the lawn whenever you want to, in order to put down a more effective plant, but once the sunshine is spent, you don't get it back.

Plants vs. Zombies is an amazingly addictive game, and the variety of gameplay is a true testament to the creativity of the designers at Popcap Games. There's so much more to say than I have room for, but the final word is this: the game is $19.95. Available for both Mac and PC, go pick it up right now.