There's a handful of carefully judged complicating factors: different seeds have different recharge times before they can be planted again, and there's a day-and-night cycle which forces you to swap between two arsenals, one composed of plants and one of fungi, every few rounds. As the game progresses and your seed collection builds, a lot of the strategy comes down to sizing up the enemy you'll be facing - you're always afforded a gentlemanly glimpse of the drooling horrors to come - and choosing the correct load-out of tools to take them down.
That in itself would probably be enough, but, as any gardener will tell you, the real pleasure lies in the sheer variety an ecology offers, and so it is with Plants vs. Zombies. Every new enemy forces you out of your comfort zone, while each additional weapon promises a fresh strain of mischief, or a tactic you simply hadn't considered yet.
The invention is dazzling. From Zomboni drivers who leave a trail of ice in their wake, and jack-in-the-box zombies bearing explosive gifts, to Gatling gun Pea Shooters and Doom Shrooms, which explode in highly localised atomic blasts, each new element is a lovely piece of punning design, drawn with thick marker pen appeal. And the game is as pacy as it is charming: as soon as a balloon zombie nonchalantly floats over your defences, you'll need to start thinking about leaving space to plant emergency Cacti or Blowers to see them off, and when nightfall closes in for the first time, you'll quickly have to work out how to keep the sunlight trickling into the bank while you take on the enemy with - initially - underpowered mushrooms.
A second lope through the knockabout apocalypse reveals that the game can handle a wide range of strategies, allowing for players who like to conserve funds and purchase the big guns, as well as those who'd rather risk living hand-to-mouth, investing in cheap one-shot deals like the potato mine and the Wall-Nut, which gives foes something to chew on, while weaker weapons whittle away at their health. Gentle terrain variations force you to rethink old ideas, as the battle moves from the front lawn to the back, and then onwards and upwards, and you're asked to contend with obstacles like a two-lane swimming pool and some nasty sprouting tombstones.
As with games like Peggle and Bejeweled, PopCap has tested and retested the learning curve until it's so perfectly tuned it's almost creepy, setting a pace that's constantly challenging but rarely too punitive, with a handful of wild-card moments to vitalise each encounter, like the sudden heavy-attack waves that are strung through every level. Elsewhere, the brilliantly simple sunlight-collection idea ensures that, from the very start, you're always multi-tasking between laying defences and collecting funds, and you can never simply sit back and watch events unfold as you can do in so many other tower defence games.
It's for reasons such as these that the tense and always adorable Plants vs. Zombies is a masterful combination of serious strategy and cartoonish delights - and by adding mini-games, survival modes and a shop, PopCap is practically rubbing it in. The result is as fresh and accessible as Super Mario, and as refined and considered as Left 4 Dead, wading into another established genre and polishing the central ideas in a way that will make it a hard act to follow. Despite the undead, you'll still be able to sleep with the lights off, but the midnight oil will be burning long and hard at the developer's increasingly frustrated competitors.
Plants vs. Zombies is available now from PopCap's website for GBP 14.95.